Sensitized CD8+ T cells fail to control organism burden but accelerate the onset of lung injury during Pneumocystis carinii pneumonia

While CD8(+) cells have been shown to contribute to lung injury during Pneumocystis carinii pneumonia (PCP), there are conflicting reports concerning the ability of CD8(+) cells to kill P. carinii. To address these two issues, we studied the effect of the presence of CD8(+) cells in two mouse models of PCP. In the reconstituted SCID mouse model, depletion of CD8(+) cells in addition to CD4(+) cells after reconstitution did not result in increased numbers of P. carinii cysts compared to the numbers of cysts in mice with only CD4(+) cells depleted. This result was observed regardless of whether the mice were reconstituted with naive or P. carinii-sensitized lymphocytes. In contrast, reconstitution with sensitized lymphocytes resulted in more rapid onset of lung injury that was dependent on the presence of CD8(+) cells. The course of organism replication over a 6-week period was also examined in the CD4(+)-T-cell-depleted and CD4(+)- and CD8(+)-T-cell-depleted mouse model of PCP. Again, the organism burdens were identical at all times regardless of whether CD8(+) cells were present. Thus, in the absence of CD4(+) T cells, CD8(+) T cells are a key contributor to the inflammatory lung injury associated with PCP. However, we were unable to demonstrate an in vivo effect of these cells on the course of P. carinii infection.