ANALYSIS OF CYTOKINE MESSENGER-RNA PROFILES IN THE LUNGS OF PNEUMOCYSTIS CARINII-INFECTED MICE

Severe combined immunodeficient (scid) mice lack functional CD4(+) lym phocytes, and therefore develop life-threatening Pneumocystis carinii infection. However, when scid mice are immunologically reconstituted w ith spleen cells, including CD4(+) cells, a protective inflammatory re sponse is mounted against the organism. To determine whether these lym phocytes induce elevated cytokine mRNA levels in response to P. carini i infection, steady-state levels of cytokine mRNAs were measured in th e lungs of both reconstituted and unaltered scid mice. Despite signifi cant numbers of organisms and the presence of functional alveolar macr ophages in the lungs of 8- and 10-wk-old scid mice, there was neither evidence of pulmonary inflammation, nor increased proinflammatory cyto kine expression. However, when 8-wk-old scid mice were immunologically reconstituted, signs of intense, focal pulmonary inflammation were ob served, and levels of interleukin (IL)-1 alpha, IL-1 beta, IL-3, IL,-6 , interferon-gamma (IFN-gamma), tumor necrosis factor (TNF)-alpha, and TNF-beta mRNAs were all significantly elevated. Cytokine expression w as increased at day 10 post-reconstitution (PR), maximal at day 12 PR, and returned to baseline by day 22 PR. In situ hybridization demonstr ated that at day 12 PR, increased IL-1 beta and TNF-alpha. expression was localized to sites of intense inflammation and focal P. carinii co lonization. Many of the cells expressing high levels of IL-1 beta and TNF-alpha in these regions were in direct contact with organisms, or c ontained degraded organisms within their cytoplasm. Thus, even though functional macrophages are present in scid mice, CD4(+) T cells are re quired for proinflammatory cytokine expression, which is associated wi th the generation of a protective inflammatory response at sites of P. carinii infection.