(A) Comparable surface expression of IL-6Rα and gp130 in the plasma membrane of B220⁺IgA⁺ B cells from 18-month-old WT and Cd37^–/– mice analyzed by flow cytometry (specific antibody staining [black] versus isotype control antibody [gray]). MFI, mean fluorescence intensity. (B) Protein levels of SOCS3 in mLN cells from 18-month-old WT and Cd37^–/– mice, as detected by Western blot (noncontiguous lanes from the same gel). SOCS3 protein levels were normalized to actin levels (n = 5 mice per genotype). Data represent mean ± SEM. (C) HEK293 cells were cotransfected (TF) with CD37-GFP (or GFP alone) and SOCS3, followed by immunoprecipitation with anti-GFP or isotype control IgG. Western blots were probed with anti-SOCS3 (26 kDa) and anti-GFP (CD37-GFP, 60–75 kDa, due to heavy glycosylation, ref. 66; noncontiguous lanes from the same gel). (D) Colocalization of CD37 (red) and SOCS3 (green) or control antibody was studied on human JY B cells without (–) or with (+) IL-6 stimulation by confocal microscopy. Colocalization is shown in yellow (white arrows). Manders coefficient was determined with n ≥ 17 cells from 3 independent experiments (each dot represents a single cell). Data represent mean ± SEM. *P < 0.05, Mann-Whitney nonparametric test. (E) Colocalization of CD37 (red) and IL-6Rα (green) or control antibody was studied on human JY B cells by confocal microscopy. Colocalization (yellow) was independent of IL-6 stimulation. Data are shown for B cells without IL-6 stimulation. (F) p-STAT3 (76–78 kDa) levels in NALM-6 cells, which were mock transfected or transfected with CD37-GFP, with or without IL-6 stimulation for 60 minutes. Actin (42 kDa) was used as loading control (noncontiguous lanes from the same gel). (G) Cd37^–/–xIl6^–/– mice are protected against development of B cell lymphoma (n = 26) (see Supplemental Table 1 for details). Original magnification: ×630 (D and E); ×1,260 (insets, D and E).