Severe combined immunodeficient (scid) mice lack functional CD4⁺ lymphocytes, and therefore develop life-threatening Pneumocystis carinii infection. However, when scid mice are immunologically reconstituted with spleen cells, including CD4⁺ cells, a protective inflammatory response is mounted against the organism. To determine whether these lymphocytes induce elevated cytokine mRNA levels in response to P. carinii infection, steady-state levels of cytokine mRNAs were measured in the lungs of both reconstituted and unaltered scid mice. Despite significant numbers of organisms and the presence of functional alveolar macrophages in the lungs of 8- and 10-wk-old scid mice, there was neither evidence of pulmonary inflammation, nor increased proinflammatory cytokine expression. However, when 8-wk-old scid mice were immunologically reconstituted, signs of intense, focal pulmonary inflammation were observed, and levels of interleukin (IL)-1α, IL-1β, IL-3, IL-6, interferon-γ (IFN-γ), tumor necrosis factor (TNF)-α, and TNF-β mRNAs were all significantly elevated. Cytokine expression was increased at day 10 post-reconstitution (PR), maximal at day 12 PR, and returned to baseline by day 22 PR. In situ hybridization demonstrated that at day 12 PR, increased IL-1β and TNF-α expression was localized to sites of intense inflammation and focal P. carinii colonization. Many of the cells expressing high levels of IL-1β and TNF-α in these regions were in direct contact with organisms, or contained degraded organisms within their cytoplasm. Thus, even though functional macrophages are present in scid mice, CD4⁺ T cells are required for proinflammatory cytokine expression, which is associated with the generation of a protective inflammatory response at sites of P. carinii infection.