KBMA Listeria monocytogenes is an effective vector for DC-mediated induction of antitumor immunity

Vaccine strategies that utilize human DCs to enhance antitumor immunity have yet to realize their full potential. Approaches that optimally target a spectrum of antigens to DCs are urgently needed. Here we report the development of a platform for loading DCs with antigen. It is based on killed but metabolically active (KBMA) recombinant Listeria monocytogenes and facilitates both antigen delivery and maturation of human DCs. Highly attenuated KBMA L. monocytogenes were engineered to express an epitope of the melanoma-associated antigen MelanA/Mart-1 that is recognized by human CD8⁺ T cells when presented by the MHC class I molecule HLA-A*0201. The engineered KBMA L. monocytogenes induced human DC upregulation of costimulatory molecules and secretion of pro-Th1 cytokines and type I interferons, leading to effective priming of Mart-1–specific human CD8⁺ T cells and lysis of patient-derived melanoma cells. KBMA L. monocytogenes expressing full-length NY-ESO-1 protein, another melanoma-associated antigen, delivered the antigen for presentation by MHC class I and class II molecules independent of the MHC haplotype of the DC donor. A mouse therapeutic tumor model was used to show that KBMA L. monocytogenes efficiently targeted APCs in vivo to induce protective antitumor responses. Together, our data demonstrate that KBMA L. monocytogenes may be a powerful platform that can both deliver recombinant antigen to DCs for presentation and provide a potent DC-maturation stimulus, making it a potential cancer vaccine candidate.

Melanoma is one of the most rapidly growing cancers worldwide, yet there is no satisfactory treatment either in the adjuvant setting or when it has advanced to metastatic disease. Several approaches to boost antitumor immune responses in melanoma have been attempted (1). While a few of these have successfully elicited high frequencies of melanoma-associated antigen–reactive T cells (2–4), their induction does not correlate with tumor regression. Complex and tailored therapies (e.g., adoptive T cell therapy or administration of anti-CTLA antibodies), have shown some success in reducing high tumor burden (5–7), albeit with high-grade toxicity. Therefore, alternative approaches are needed.

DCs are attractive cellular adjuvants that can increase host resistance to tumors (8). To optimally prime naive T cells, DCs must not only present high levels of MHC-peptide complexes but must also undergo activation and maturation. An ideal DC-targeting vaccine formulation, therefore, should simultaneously deliver antigens and activate DCs to enhance expression of surface MHC-peptide complexes and costimulatory molecules and production of cytokines that prime potent CD8⁺ and CD4⁺ T cell responses. To date, there is no standard, efficient, and cost-effective method that combines antigen loading and activation/maturation of DCs for clinical immunotherapy.

Listeria monocytogenes are Gram-positive facultative intracellular bacteria that are internalized by phagocytic and nonphagocytic cells. Previous studies have shown that self or heterologous antigens encoded by L. monocytogenes or the recombinant L. monocytogenes strains can access endogenous and exogenous antigen processing and presentation pathways to prime CD8⁺ and CD4⁺ T cells, respectively. Following internalization, L. monocytogenes gain access to the cytosol of APCs from the phagosome through expression of listeriolysin O (LLO). In addition, despite its preferential intracytoplasmic location, L. monocytogenes very efficiently prime CD4⁺ T cell responses (9). These characteristics have made L. monocytogenes an attractive and efficacious platform in murine models of cancer and microbial infection. In humans, infection with live WT or recombinant L. monocytogenes activates DCs in vitro (10); however, for broad clinical application one must address pathogenicity while preserving immunogenicity of recombinant L. monocytogenes vaccines.

The potency of killed but metabolically active (KBMA) recombinant L. monocytogenes was recently demonstrated in an animal model of tumor immunity (11). KBMA L. monocytogenes are rendered noninfectious by the combined use of psoralen and long-wave UV light (UVA) and abrogation of nucleotide excision repair through deletion of the bacterial uvrAB genes. While KBMA L. monocytogenes cannot propagate, they remain metabolically active and express their genetic repertoire, including the heterologous antigens they were engineered to express (11). Most importantly, KBMA L. monocytogenes induced cytolytic CD8⁺ T cell responses to an endogenous tumor epitope and reduced tumor burden in a therapeutic CT-26 tumor model (10).

Several murine studies have provided us with evidence of the fundamental relationship between L. monocytogenes and endogenous DCs during an infection. Priming of naive CD8⁺ T cells to L. monocytogenes–specific antigens is mediated by DCs (12). Contrary to previous assumptions, DCs are also required to maximally activate a recall response to various pathogens, including L. monocytogenes (13). These 2 events may be related to the fact that DCs are necessary for the efficient entry of L. monocytogenes into the spleen, in which T cell activation takes place (14). All together, this makes L. monocytogenes a promising candidate for development of human DC-targeting vaccines.

We report here that KBMA recombinant L. monocytogenes can be used as an antigen-loading vector to target human DCs. KBMA L. monocytogenes encoding tumor antigens induce DC maturation, secretion of Th1-polarizing cytokines and type I interferons, and expression of the recombinant tumor antigen and confer potent antigen priming activity to these APCs. Once infected with KBMA L. monocytogenes expressing full-length NY-ESO-1 protein, DCs presented antigens in the context of both class I and class II MHC molecules by donors with different MHC haplotypes. Furthermore, antigen-primed CD8⁺ T cells acquired effector function and lysed a patient-derived melanoma tumor cell line. Thus, recombinant L. monocytogenes are a unique antigen delivery vehicle that can mature DCs for T cell priming, while simultaneously delivering both CD8⁺ and CD4⁺ T cell–restricted epitopes for presentation, independently of patient’s MHC restriction.

Deletion of selected virulence proteins preserves the DC activation potential of L. monocytogenes. The ability of L. monocytogenes to infect phagocytic cells, as well as to access the host cell cytosol following the escape from the phagolysosome, renders these bacteria a suitable vaccine platform for the delivery of antigens into the endogenous and exogenous pathways of antigen processing by DCs. It has been reported that WT L. monocytogenes (i.e., fully infectious and replicating L. monocytogenes) are extremely efficient at activating human monocyte-derived DCs (moDCs) (10). Their use, however, is contraindicated in humans. Therefore, we first tested whether highly attenuated strains of L. monocytogenes, which are suitable for clinical applications, retain their ability to efficiently activate DCs in vitro. The bacterial genes actA, inlB, and hly encode 3 important L. monocytogenes virulence determinants: ActA, which mediates actin polymerization; internalin B (inlB), which facilitates cell binding and invasion; and pore-forming LLO, which permits escape from phagolysosomes to cytosol. L. monocytogenes strains attenuated by deletion of either both the actA and inlB genes (ΔactAΔinlB L. monocytogenes) or the hly gene (ΔLLO L. monocytogenes) were compared with WT L. monocytogenes in their ability to stimulate DCs to upregulate maturation markers, secrete proinflammatory cytokines, and stimulate allogeneic T cells. As anticipated, presence of LLO enhanced activation of DCs (Figure 1). Confirming previously reported data (15), deletion of the hly gene, to some extent, impaired the ability of DCs to upregulate the costimulatory molecules CD80 and CD83 (Figure 1, A and B) as well as the proinflammatory and pro-Th1 cytokines IL-1β (data not shown), TNF-α, and IL-12p70 (Figure 1C). However, the difference was only observed at lower MOI. Activation of DCs with ΔactAΔinlBL. monocytogenes did not statistically significantly impair DC maturation when compared with the parent strain (Supplemental Table 1; supplemental material available online with this article; doi: 10.1172/JCI31350DS1)). In accordance with these findings, DCs infected with any of the attenuated strains induced proliferation of allogeneic T cells comparable to control DCs matured with a cocktail of inflammatory cytokines (a combination of IL-6, IL-1β, TNF-α, and PGE₂ hereafter referred to as MC) (Figure 1D). Thus, the ability to induce DC maturation was not impaired with the live-attenuated L. monocytogenes strain deleted of 2 virulence factors ActA and InlB.

Figure 1

Deletion of virulence factors and immunogenicity of L. monocytogenes. Live-attenuated L. monocytogenes strains (Lm) deleted of either hly (ΔLLO) or actA and inlB (ΔactAΔinlB) were compared with WT L. monocytogenes, LPS, unstimulated (Imm), or a cocktail of inflammatory cytokines (MC) in their potency to mature human moDCs. (A and B) DCs were harvested 40 hours after infection with L. monocytogenes (MOI 1), and expression of CD80 and CD83 was assessed by flow cytometry. (A) Data for a single donor (B) and median value (horizontal line) and range (vertical line) with 25th and 75th percentiles (bars) of at least 7 individual results are shown. (C) At the same time supernatants from control and L. monocytogenes–infected DC cultures were collected (white bars, MOI 0.1 or controls; gray bars, MOI 1; black bars, MOI 10) for quantitation of IL-12p70 and TNF-α by cytokine bead array assay. Results of 3 representative donors are shown. (D) DCs were generated as above and cultured with allogeneic naive CD4⁺ T cells for 4 days. T cell proliferation was measured by incorporation of radioactive thymidine during the last 16 hours. A representative experiment of 3 is shown.

KBMA L. monocytogenes are stimulatory for human DCs. While heat-killed (HK) vaccines are safer to use, this is often at the expense of their immunogenicity. To further improve safety, we engineered a KBMA L. monocytogenes vaccine candidate on the ΔactAΔinlBΔuvrABL. monocytogenes background as described previously (11). KBMA L. monocytogenes could no longer replicate, but they maintained metabolic activity, retained the ability to exit the phagolysosome, and escaped into the cytosol only where they colocalized with actin (Figure 2A). Therefore, recombinant antigen-expressing KBMA L. monocytogenes, in contrast to killed L. monocytogenes, have the potential to deliver antigens to the endogenous antigen processing pathway, a necessity for a successful vaccine vector, without the associated risks of a replicating live bacterium. The KBMA L. monocytogenes strain was compared with live, killed, and metabolically nonactive L. monocytogenes (inactivated with a high synthetic psoralen DNA cross-linking agent [S-59] concentration) with the same genetic background (Figure 2 and Supplemental Figure 1). The ability of KBMA L. monocytogenes to activate DCs was compared with well-described maturation stimuli LPS, poly I:C; or MC. Live ΔactAΔinlBΔuvrABL. monocytogenes were as effective as 2 positive controls (LPS and MC) in terms of activating DCs (upregulation of typical maturation markers; Figure 2, B and C). Furthermore, the KBMA L. monocytogenes vaccine strain induced maturation of DCs to a similar extent as live bacteria, provided a higher MOI was used (10 vs. 1 MOI) (Figure 2, B and C). A similar pattern was observed with cytokines secreted by DCs upon infection (Figure 2, D–F). Live, KBMA L. monocytogenes, and LPS induced TNF-α, IL-6, and IL-12p70 to comparable levels and live and KBMA L. monocytogenes induced these to higher levels than HK L. monocytogenes (Figure 2D). Similar results were obtained with CD11c⁺ DCs isolated from blood of healthy donors (Supplemental Figure 2). KBMA and live L. monocytogenes were equivalent inducers of MCP-3 and RANTES, 2 chemokines that recruit T cells (Figure 2E).

Figure 2

KBMA L. monocytogenes activate moDCs. (A) DCs were infected with L. monocytogenes strains, stained with antibodies to L. monocytogenes (green), phalloidin (red), and DAPI (blue). Visualization of polymerized actin and colocalization with the bacteria (yellow) confirms the presence of bacteria within the cytoplasm. Scale bar: 10 μm. (B and C) DCs were infected with live (L), HK, or KBMA L. monocytogenes. Unstimulated or DCs, exposed to LPS or MC, were used as controls. Expression of DC-maturation markers for a typical donor is shown (gray bars, MOI 1; black bars, MOI 10) (B). Results of multiple donors are shown in C (MOI 10). (D and E) Presence of cytokines (D) and chemokines (E) was measured by ELISA (gray bars, MOI 1; black bars, MOI 10). (F) DCs were infected at MOI 10 (WT, L) or MOI 200 (ΔLLO, HK, and KBMA L. monocytogenes). Immature, LPS, or poly I:C–stimulated DCs were used as controls. IFN-α in supernatants was measured. (G) DCs were infected with live, HK, or KBMA L. monocytogenes and left untreated or stimulated with LPS or MC. CCR7 was measured by flow cytometry after 40 hours. (H) DCs were infected with live or KBMA L. monocytogenes. CCR7 was measured at indicated time points. Numbers in parentheses indicate MOI used for infection. Nonstimulated or LPS-stimulated DCs were used as controls. In C, F, and G, values from individual donors (dots), mean values (lines), and Student’s t test P values are shown. In A, B, D, E, and H, a representative experiment of at least 3 performed is shown. In D and E, error bars represent SD of triplicate culture wells.

WT L. monocytogenes but not HK L. monocytogenes induce type I IFN transcription and secretion in mouse DCs (16). Similarly, secretion of type I IFN-α has also been detected following infection of murine macrophages with WT but not ΔhlyL. monocytogenes strains (17). As shown in Figure 2F, we found that substantial amounts of IFN-α were produced by human moDCs upon exposure to live and KBMA L. monocytogenes but not HK or ΔhlyL. monocytogenes, confirming the requirement for cytoplasmic entry of bacteria.

CCR7 is a chemokine receptor that recognizes the chemokines CCL19 and CCL21 produced in lymphatic vessel endothelial cells and T cell areas of lymph nodes and is therefore essential for trafficking of DCs to lymph nodes. Flow cytometry analysis revealed that infection with live and KBMA L. monocytogenes induced expression of CCR7 on DCs (Figure 2, G and H). The onset of CCR7 expression was rapid, commencing 6 hours after infection, with high levels maintained 24 hours after infection. The levels and kinetics of expression of CCR7 were comparable among LPS-matured DCs or DCs infected with live or KBMA L. monocytogenes strains and were superior to HK L. monocytogenes–infected DCs. Altogether, our data indicate that the coincubation of the KBMA L. monocytogenes vaccine strain with human DCs leads to full maturation of DCs and optimally equips them for priming.

KBMA L. monocytogenes encoding recombinant antigens access the endogenous pathway and stimulate antigen-specific CD8⁺ T cells. To test the ability of KBMA bacteria to load antigen into the MHC class I pathway of APCs, ΔactAΔinlBΔuvrABL. monocytogenes were constructed to express HLA-A*0201–restricted CD8⁺ T cell epitopes of influenza matrix protein (MP) and melanoma-associated antigen MelanA/Mart-1 protein (LmAg). DCs infected with KBMA LmAg at MOI 100 were compared with DCs infected with live LmAg (MOI 10) (Figure 3A). DCs infected with empty vector served as negative controls. Forty hours after infection, the DCs were washed and cocultured with MP_58–66- or MART1_26–35-specific CD8⁺ T cell clones for 24 hours. The number of cells synthesizing IFN-γ was enumerated by enzyme-linked immunospot (ELISPOT) assay. It should be pointed out that in this assay the number of responding T cell clones depends on multiple factors and not every clone will respond, even when DCs are optimally loaded with antigens (18). Activation of antigen-specific T cell clones was observed with HLA-A*0201⁺ DCs infected with either live or KBMA LmAg, demonstrating that both live or KBMA L. monocytogenes can deliver antigen into the MHC class I pathway of antigen processing. No responses were observed either when HLA-A*0201⁺ donor DCs were infected with the empty vector or when HLA-A*0201^– donor DCs were infected with LmAg, thus confirming antigen specificity. Replication was not necessary for the presentation of recombinant antigens, since KBMA L. monocytogenes strains were able to induce effector responses from antigen-specific T cells. Both live and KBMA L. monocytogenes induced marked responses from T cells as early as 6 hours after infection. DCs infected with KBMA L. monocytogenes sustained expression over the first 12 hours, while those infected with live L. monocytogenes showed an increase in expression over time, likely related to replication of bacteria within the infected cell (Figure 3B). In both groups, expression of heterologous antigen remained detectable as demonstrated by the activation of antigen-specific T cells up to 40 hours after infection (Figure 3A). This period of antigen expression and recognition would encompass the time required for DCs to prime effector T cells in vivo (19).

Figure 3

Live and KBMA L. monocytogenes successfully deliver recombinant antigens for presentation by MHC I molecules. (A) DCs from HLA A2.1⁺ or HLA A2.1^– donors were infected with recombinant L. monocytogenes ΔactAΔinlBΔuvrAB strain expressing LmAg or empty vaccine vector (L. monocytogenes) at MOI 10 (live) or MOI 100 (KBMA). After 40 hours, DCs were cocultured with 20,000 MP_58–66 or Mart-1_26–35–specific CD8⁺ T cell clones in an IFN-γ–capturing ELISPOT assay. The number of spots corresponding to IFN-γ secreting T cells (SFC) is shown. Black bars represent responses of T cell–DC cocultures, and white bars represent DC-only cultures. Error bars represent SD of triplicate culture wells. (B) DCs were infected with live or KBMA LmAg (left panel) at the indicated MOI in 96-well plates. As a control, we used noninfected DCs, either unpulsed or pulsed with MP_58–66 peptide or DCs infected with L. monocytogenes that were either unpulsed or pulsed with MP_58–66 (right panel). At indicated times, DCs were fixed and MP-specific T cells were added to the wells. After 18 hours of T cell–DC coculture, supernatants were harvested and the amount of IFN-γ produced by T cells was determined by ELISA.

KBMA L. monocytogenes encoding full-length NY-ESO-1 protein delivers antigens for presentation by MHC I and MHC II molecules. Ideally, cancer vaccines should prime both CD4⁺ and CD8⁺ T cells. Immunogenic epitopes have been identified for many cancer-relevant antigens, mostly for HLA A2.1–specific CD8-restricted epitopes, while CD4-restricted epitopes have been identified for various MHC haplotypes (20, 21). When individual CD4- and CD8-restricted epitopes are combined into vaccines, only a very small group of patients that receive these treatments have the required combination of MHC class I and class II molecules to elicit desired responses. Thus, whole proteins are a better alternative, especially when targeted specifically to professional APCs. NY-ESO-1 is a cancer-testis antigen, detectable in up to 30%–40% of metastatic melanomas. It is expressed by a variety of cancers but not in adult somatic tissues, making it an attractive target for immunotherapies (22–24). We have shown previously that when administered with adjuvants, NY-ESO-1 protein induces both humoral and cellular immune responses (25). Here we tested whether KBMA attenuated L. monocytogenes that express the NY-ESO-1 protein are an efficient adjuvant and delivery system.

For the purpose of the experiment, we first generated NY-ESO-1–specific CD8⁺ and CD4⁺ T cell clones. The CD8⁺ clones were HLA A2.1 restricted while CD4⁺ clones were DR (clone no. 15, clone no. 18) or DQ (clone no. 30) restricted. HLA A2.1⁺ or HLA DR B4*0101⁺ DCs were infected either with KBMA ΔactΔinlBΔuvr L. monocytogenes encoding full-length NY-ESO-1 protein (LmNY) or its empty vector control (L. monocytogenes). CD4⁺ or CD8⁺ NY-ESO-1–specific T cell clones were then added for overnight culture, after which IFN-γ in supernatants was measured. As shown in Figure 4, KBMA LmNY-infected DCs activated both NY-ESO-1–specific CD4⁺ and CD8⁺ T cell clones. While CD8⁺ T cell responses were comparable among the 6 donors tested, CD4⁺ T cell responses varied in magnitude in a donor-specific manner. However, all observed responses were NY-ESO-1–specific and statistically significant when compared with responses elicited by empty vector (Figure 4 and Supplemental Figure 3A). Interestingly, DCs infected with KBMA L. monocytogenes elicited a more pronounced response from the NY-ESO-1–specific CD8⁺ T cell clones than the DCs infected with live L. monocytogenes when both strains were used at the highest MOI, which does not yet to a large extent impair DC viability (MOI 100 vs. MOI 10). To confirm that presentation of NY-ESO-1 epitopes was not limited to in vitro generated DCs but would take place also when heterogenous APC populations are targeted, PBMCs or monocytes were isolated from blood, infected with LmNY, and used as stimulators for NY-ESO-1–specific T cell clones. These infected APCs were DR or DQ restricted and were cocultured with HLA matched clones overnight when we evaluated NY-ESO-1–specific clonal responses by measurement of IFN-γ in culture supernatants (Supplemental Figure 3, B and C). We confirmed that in addition to moDCs, heterogenous PBMC populations and monocytes isolated from blood efficiently processed and presented the full-length NY-ESO-1 protein encoded by L. monocytogenes.

Figure 4

LmNY deliver antigens for presentation by MHC class I and class II molecules. Following infection with NY-ESO-1 protein encoding ΔinlBΔactAΔuvrAB L. monocytogenes (LmNY) or the empty vector control (L. monocytogenes) DCs were cocultured with either CD4⁺ or CD8⁺ NY-ESO-1–specific T cell clones for 18 hours. Supernatants were collected and IFN-γ was measured in an ELISA assay. Background IFN-γ secretion was determined by DC-only culture. Error bars represent SD of quadruplicate wells of DCs or T cell–DC culture. (A) HLA A2.1⁺ DCs were infected and cocultured with 2 separate HLA A2.1–restricted NY-ESO-1–specific CD8⁺ T cell clones. MOI for infection was 10 for live and 100 for KBMA L. monocytogenes. The experiment is a representative of 6 different donors tested. (B) DCs derived from 2 HLA DR B4*0101 restricted donors (shown are Donors 1 and 2 out of 3 donors tested) were infected with KBMA L. monocytogenes at MOI 100 and cocultured with NY-ESO-1–specific HLA DR B4*0101–restricted CD4⁺ clone 15.

In summary, our data demonstrate that both CD8- and CD4-restricted epitopes are efficiently processed and presented to T cell clones by APCs when the full-length protein is delivered by the L. monocytogenes vector.

KBMA L. monocytogenes–infected DCs prime melanoma antigen-specific effector CD8⁺ T cells. To be effective in vivo, KBMA L. monocytogenes vectors must confer antigen-specific T cell priming activity to DCs. Therefore, naive T cells from a healthy individual were cocultured with autologous DCs after infection with KBMA LmAg encoding the Mart-1 epitope. Every 9 days, the T cells were restimulated with KBMA LmAg–infected DCs for a total of 3 stimulations. As shown in Figure 5A, DCs infected with KBMA LmAg expanded MP_58–66–specific naive CD8⁺ T cells. Before the onset of priming, no staining of naive T cells was apparent with visualization of tetrameric complexes of HLA-A*0201 and either MP_58–66 (Figure 5A, left panel) or Mart-1_26–35 (data not shown), verifying that the starting T cell population did not contain significant numbers of MelanA/Mart-1– or MP-specific cells.

Figure 5

Recombinant L. monocytogenes prime effector CD8⁺ T cells to the encoded recombinant antigens. HLA A*0201⁺ DCs were infected with the KBMA ΔactAΔinlBΔuvrABL. monocytogenes strain expressing the HLA A*0201–restricted CD8⁺ T cell epitopes of MP and MelanA/Mart–1. (A) The naive T cell population lacks the MP-tetramer–positive cells (left panels). After stimulation, MP-specific CD8⁺ T cells were generated at various frequencies (each middle panel representing a single well). Lack of staining with HIV Gag tetramers assured specificity (right panels). The numbers indicate the percentage of tetramer-positive cells. (B) Both live and KBMA recombinant L. monocytogenes primed naive Mart-1_26–35–specific T cells at similar frequencies. Percentage of CD3-gated, CD8, and tetramer double-positive cells after priming (left panels) or after the enrichment with limiting dilution assay is shown (right panels). HIV Gag tetramers were used as a negative control. The population of cells enriched for Mart-1–tetramer-positive cells was used for the functional assays shown in C–E. (C and D) T cells were stimulated with T2 cells pulsed with Mart-1_26–35 peptide, and Mart-1–tetramer-positive cells were analyzed for secreted cytokines by intracellular staining. HIV Gag-tetramers were used as specificity controls. In C, the dot plots of 1, and in D, pie charts representing the spectrum of secreted cytokines by 2 tested donors are shown. In C, the numbers indicate the percentage of cells in each dot plot quadrant. (E) The same T cell populations were tested for cytotoxicity of melanoma cell lines. Gmel and 888 melanoma cell lines express MelanA/Mart-1 protein (left panel), but only Gmel is HLA A*0201–restricted (middle panels), which correlates with the latter’s ability to be lysed by MelanA/Mart-1–specific T cells (right panel). Original magnification, ×100. Preincubation of Gmel with anti-HLA A*0201 antibodies inhibited lysis. A representative result for 1 of 2 donors is shown in A–C and E. In E, error bars represent SD of triplicate culture wells.

We next evaluated whether live LmAg was more efficient at priming naive T cells to Mart-1_26–35 than KBMA LmAg in our in vitro system. DCs infected with live LmAg (MOI 10) or KBMA LmAg (MOI 50) were used to stimulate T cells as described above, and priming efficacy was tested with Mart-1_26–35 tetramers. A higher MOI was used for KBMA LmAg to compensate for this strain’s inability to replicate inside the infected cell. Both strains were able to prime Mart-1_26–35-specific T cells at similar frequencies (Figure 5B, 4 left panels). The Mart-1_26–35 population from the culture stimulated by KBMA LmAg–infected DCs was enriched by using a limiting dilution assay of pooled wells in which Mart-1_26–35 tetramer-positive cells were detected (Figure 5B, 2 right panels). Specific antigen responses were measured by intracellular cytokine secretion. Only Mart-1_26–35 tetramer-positive cells responded to stimulation with Mart-1_26–35 peptide-pulsed T2 cells but not to HLA-A*0201–restricted HIV Gag_77–85 peptide-pulsed T2 cells that we used as a negative control (Figure 5C). In 2 different donors, most of the responding Mart-1_26–35 tetramer-positive cells produced several cytokines, including IFN-γ, TNF-α, and IL-2 (24% and 17% of Mart-1_26–35 tetramer-positive cells produced all 3 cytokines in both donors) (Figure 5D). Cytotoxicity of the Mart-1_26–35–specific T cells primed with KBMA LmAg was tested by using a chromium release assay. These cells lysed the MelanA/Mart-1 expressing A2.1⁺ cell line Gmel but not the A2.1-888 melanoma line (Figure 5E). The lysis of Gmel was blocked when cells were pretreated with an anti-A2.1 antibody. Our data show that CD8⁺ T cells primed by KBMA LmAg–infected DCs recognize native Mart-1 peptide and are able to execute their effector functions to destroy a melanoma tumor cell line.

In a murine therapeutic tumor model, KBMA L. monocytogenes are more efficient as a vaccine if targeting APCs in vivo. In terms of targeting DCs for therapeutic treatment of tumors, there are 2 main strategies that can be employed. DCs can be matured and loaded with antigens ex vivo and then injected back into patients. This approach is well established, although DC-based immune therapies remain to be fully optimized. The second approach is to target DCs in vivo, either directly or via intermediate vector “carrier cells” that are cross-presented by DCs. Apart from being simpler and cheaper, this approach offers the advantage of targeting multiple DC types in addition to other APCs. Using L. monocytogenes as a vaccine vector makes the second approach even more appealing, since upon i.v. injection, L. monocytogenes spontaneously localize in T cell priming areas of the spleen (26) and deliver antigens for priming by resident DCs (12). It was shown previously that targeting APCs in vivo with KBMA L. monocytogenes encoding tumor antigen mediates priming to tumor antigens, breaks tolerance to the endogenous antigens, and controls tumor growth (11). We have now used a therapeutic vaccination model in CT-26 tumor-bearing mice. We have compared in vivo targeting to ex vivo loading of DCs using KBMA L. monocytogenes expressing the immunodominant T cell epitope AH1/A5. While ex vivo loading of DCs with KBMA L. monocytogenes–expressing AH1/A5 elicited a small AH1/A5-specific T cell response, the protective effect of the primed cells was insignificant (Figure 6). Both, DCs loaded with vaccine or empty vector mediated similar transient control of tumor growth compared with the untreated mice, likely dependent on stimulation of innate immunity. On the contrary, therapeutic immunization of mice with either live or KBMA recombinant L. monocytogenes induced protective immune responses against the tumor that expresses endogenous antigen (Figure 6B) and significantly prolonged survival when mice were followed for 100 days. In summary, KBMA L. monocytogenes expressing tumor antigen is a potent vaccine vector that primes protective T cell responses in mice, when its recombinant antigen is delivered to the in vivo resident APCs.

Figure 6

Targeting DCs with KBMA L. monocytogenes–expressing AH1/A5 in vivo controls CT-26 tumor growth. (A) Five mice per group were used for assessment of priming efficacy. The efficacy of T cell priming was compared between mice infected in vivo (left panel) and mice vaccinated with DCs infected in vitro (right panel). Mice were sacrificed 7 days after immunization, and the percentage of antigen-positive IFN-γ–secreting CD8⁺ T cells upon in vitro stimulation with AH1 or AH1/A5 peptides in the presence of brefeldin A was analyzed by flow cytometry. Error bars represent SD of triplicate culture wells. (B) BALB/c mice were inoculated with CT-26 tumors and immunized 5 days later. They were randomized (10 per group) and vaccinated either with 1 of the indicated DC vaccines or with the live or KBMA L. monocytogenes–expressing AH1/A5. Survival of the mice was monitored over 100 days. P value of survival difference in mice vaccinated with DCs pulsed ex vivo with L. monocytogenes–expressing AH1/A5 versus mice infected with L. monocytogenes–expressing AH1/A5 is shown (P = 0.048). No significant difference was observed among mice vaccinated with live-attenuated or KBMA L. monocytogenes (P = 0.328).

Current therapeutic strategies to treat metastatic cancer have either shown limited success in tumor regression or were associated with high toxicities. Therefore, alternative approaches are urgently needed. DCs are attractive adjuvants in humans to increase host resistance to tumors. DCs acquire antigens through several mechanisms, efficiently process and present antigens to T cells (8), and have an excellent safety record in humans. However, new strategies are needed to optimize their immunogenicity, as there have been few reports demonstrating their ability to induce sustained, protective antitumor immunity in humans (27, 28). One such strategy is to use a system that efficiently targets DCs in vivo. In addition to possibly targeting more relevant in vivo resident DC subsets as well as other APCs, such strategies would also considerably lower the cost of the vaccine.

L. monocytogenes are one of the recombinant microbial vector platforms that has proved to elicit protective responses in a number of experimental models, including infectious and malignant diseases (29–31). L. monocytogenes localize to T cell zones of spleen white pulp and efficiently targets DCs in vivo (26). Development of T cell anti–L. monocytogenes immunity depends on the presence of CD11c⁺ cells such as DCs (12, 26). Next to its remarkable immunogenicity and cost-effectiveness, several other features make L. monocytogenes an intriguing tumor vaccine candidate. These include the ability to break tolerance to self antigens (11, 32, 33) and the preserved efficacy of the recombinant L. monocytogenes vaccines despite preexisting immunity to the bacteria (34).

While only mild toxicity was observed when partially attenuated L. monocytogenes were administered to healthy human volunteers orally (35), it is not considered safe to administer a virulent form of a human pathogen to immunocompromised cancer patients. Efforts to reduce virulence of L. monocytogenes have focused on determinants that enable intracytoplasmic escape (LLO), replication, and intercellular spread without exposure to hostile extracellular defense components (ActA and InlB). Among the virulence determinants, LLO is likely one of the major factors in pathogenicity of L. monocytogenes; however, its deletion is also associated with a decrease in protectiveness of the T cells generated in infected mice (36).

Thus, to address the concerns inherent to delivering potentially toxic bacteria to humans, we constructed highly attenuated strains, lacking the proteins ActA and InlB, a strategy that reduces toxicity in mice by 1,000 fold (31). We show that ΔactAΔinlBL. monocytogenes can still efficiently enter human DCs through InlB-independent phagocytic mechanisms and preserve DC-stimulatory capacity through their ability to mature these APCs. To ensure the safety of using L. monocytogenes as a vaccine, a further attenuating step was taken by limiting the ability of the bacteria to replicate. While HK L. monocytogenes are safe to use in mice, the primed CD8⁺ T cells of the memory repertoire do not acquire effector functions and are not protective from subsequent L. monocytogenes infection (37). Immunization of mice with a mixture of live and HK L. monocytogenes does not alter the phenotype of the CD8⁺ T cells primed by the HK L. monocytogenes, thus the absence of an inflammatory milieu cannot alone explain this intrinsic difference (26). Several approaches have been utilized to interfere with the replication of L. monocytogenes while preserving T cell–stimulatory capacity (38, 39). For clinical application, a well-controlled, cost-effective method with consistent inactivation of L. monocytogenes and full DC-stimulatory potential is preferred. We developed our candidate strain on a recently described vaccine platform that simultaneously takes advantage of the potency of live and the safety of killed microbes (KBMA L. monocytogenes) (11). KBMA vaccine strains were constructed by abrogating the capacity for nucleotide excision repair through deletion of the UV light resistance genes uvrA and uvrB. The gene deletion of uvrAB renders the bacteria exquisitely sensitive to photochemical inactivation through the combined treatment of S-59 and UVA. Because of their inability to repair the DNA cross-links, KBMA L. monocytogenes are unable to replicate and are thus functionally noninfectious (11). The very limited number of cross-links, however, preserves their metabolic activity, ability to exit the phagolysosome, and ability to escape into the cytosol. L. monocytogenes that are inactivated by high doses of S-59, with a correspondingly increased number of cross-links per genome, have abrogated replication and metabolic activity, similar to the recently described L. monocytogenes inactivated by irradiation (38).

In our studies, the L. monocytogenes with preserved metabolic activity supercede their nonmetabolic counterparts in DC-stimulatory potential (Supplemental Figure 1). The potent maturation stimulus provided by metabolically active L. monocytogenes to DCs is likely to be multifactorial. L. monocytogenes activate APCs through ligation of TLRs and through cytoplasmic TLR-independent pathways (17, 40–42). In addition, the induction of proinflammatory cytokines such as IL-1, TNF, IL-6, and type I IFN, factors known to activate immature DCs, are likely to amplify these effects.

To prove that highly attenuated KBMA could be used to target tumor antigens to DCs, we constructed recombinant L. monocytogenes that contain the MelanA/Mart-1 epitope or L. monocytogenes expressing full-length NY-ESO-1 protein under the LLO promoter. Our studies demonstrate that the proteins are efficiently processed within the infected DCs and MelanA/Mart-1 epitopes are expressed in sufficient amounts on the surface of DCs to prime MelanA/Mart-1–specific CD8⁺ T cells. KBMA recombinant L. monocytogenes–infected DCs induced cytolytic T cells that recognized native antigen expressed by patient-derived melanoma cells and secreted a panel of effector cytokines, including IFN-γ, TNF-α, and IL-2. Along with the HLA-A*0201–restricted MelanA/Mart-1 epitope, the recombinant L. monocytogenes also expressed an MP epitope. Supporting previous observations in the murine system (34), MelanA/Mart-1 priming was not hindered due to preexisting immunity to influenza (data not shown).

Unlike most viral vectors, L. monocytogenes constructs are not neutralized by preexisting L. monocytogenes–specific immunity. In addition, L. monocytogenes are not restricted by the length of heterologous sequence that can be inserted and thus allows the encoding of full-length proteins. This offers 2 advantages: (a) development of a vaccine or immune therapeutic that circumvents the problems of HLA restriction and (b) simultaneous priming of CD4⁺ and CD8⁺ T cells. The second is especially advantageous in tumor settings where CD4⁺ T cell licensing is necessary for long-term protective CD8⁺ T cell responses (43–45). We show that L. monocytogenes expressing full-length NY-ESO-1 protein efficiently delivered antigens for both MHC class I and class II presentation across different MHC restrictions.

While ex vivo maturation and antigen loading of DCs is a well-established vaccine approach, which offers the advantage to monitor for vaccine quality before injection into patients, it has yet to prove its efficacy in patients with larger tumor burdens where tolerance to tumor antigens has been established. While prophylactic vaccination in an OVA-expressing tumor model by ex vivo loaded DCs mediated protective T cell responses to tumor (data not shown), we found that ex vivo loaded DCs had reduced ability to induce protective responses in a therapeutic vaccination model, using CT-26 tumor-bearing mice. In these experiments, initial immunization was also followed by 2 booster vaccinations. In contrast, targeting DCs in vivo by infecting mice with attenuated KBMA recombinant L. monocytogenes primed effector T cells that increased survival of tumor-bearing mice, an effect that was comparable to protection mediated by live L. monocytogenes. While there may be several reasons for the observed differences, the animal data indicate that vaccination strategies may need to be approached differently in tumor-free patients with high likelihood of recurrence versus patients with metastatic disease or unresectable tumors.

In summary, an ideal cancer vaccine would simultaneously induce CD8⁺ and CD4⁺ T cells. This can be made possible by using a vector that encodes a full-length cancer-relevant protein and preferentially targets APCs in vivo. Altogether, our studies show that KBMA ΔactAΔinlBΔuvrAB recombinant L. monocytogenes are a safe, efficient, and cost-effective method of antigen loading and activation/maturation of human DCs with great promise for successful clinical use.

View Supplemental data

Supported by grants from the National Cancer Institute (CA84512), the National Institutes of Health (AI044628, AI061684, AI057127), the Bill & Melinda Gates Foundation, the Mary Kirkland Foundation, and the Cancer Research Institute. N. Bhardwaj is a Doris Duke Distinguished Scientist and an Elizabeth Glaser Scientist. We thank David O’Neill and Sylvia Adams for providing melanoma patient PBMCs. The tetramers were prepared by the New York University Cancer Institute Vaccine and Cell Therapy Core Facility.

Address correspondence to: Mojca Skoberne or Nina Bhardwaj, New York University School of Medicine, 522 First Avenue, SML 1307, New York, New York 10016, USA. Fax: (212) 263-6729; E-mail: Mojcaskoberne@yahoo.de (M. Skoberne). Phone: (212) 263-5814; Fax: (212) 263-6729; E-mail: Nina.Bhardwaj@nyumc.org (N. Bhardwaj).

Nonstandard abbreviations used: Cr-51, Chromium-51; ELISPOT, enzyme-linked immunospot; HK, heat killed; Inl, internalin; KBMA, killed but metabolically active; LLO, listeriolysin O; LmAg, HLA-A*0201–restricted CD8⁺ T cell epitopes of MP and melanoma-associated antigen MelanA/Mart-1 protein; LmNY, L. monocytogenes encoding full-length NY-ESO-1 protein; MC, a cocktail of IL-6, IL-1β, TNF-α, and PGE₂; moDC, monocyte-derived DC; MP, influenza matrix protein; S-59, synthetic psoralen DNA cross-linking agent; UVA, long-wave UV light.

Conflict of interest: T.W. Dubensky Jr., D.G. Brockstedt, K.S. Bahjat, P. Lauer, M. Leong, W. Luckett, W. Liu, and E. Lemmens are or were employees of Anza Therapeutics, which owns intellectual property covering the compositions and methods described in this manuscript. N. Bhardwaj was a former Science Advisory Board (SAB) member of Anza Therapeutics at the time of the study design. In addition, Anza Therapeutics employees hold stock options in the company. N. Bhardwaj is a coinventor of patents pertaining to human dendritic cell preparation.

Reference information: J. Clin. Invest.118:3990–4001 (2008). doi:10.1172/JCI31350

Keith S. Bahjat’s present address is: Earle A. Chiles Research Institute, Robert W. Franz Cancer Research Center, Providence Portland Medical Center, Portland, Oregon, USA.

Mojca Skoberne’s present address is: Genocea Biosciences, Cambridge, Massachusetts, USA.

Mojca Skoberne, Alice Yewdall, and Keith S. Bahjat contributed equally to this work.

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