Immune responses toPneumocystis murinaare robust in healthy mice but largely absent in CD40 ligand-deficient mice

Abstract

Pneumocystis is a pathogen of immunocompromised hosts but can also infect healthy hosts, in whom infection is rapidly controlled and cleared. Microarray methods were used to examine differential gene expression in the lungs of C57BL/6 and CD40 ligand knockout (CD40L-KO) mice over time following exposure to Pneumocystis murina. Immunocompetent C57BL/6 mice, which control and clear infection efficiently, showed a robust response to infection characterized by the up-regulation of 349 primarily immune response-associated genes. Temporal changes in the expression of these genes identified an early (Week 2), primarily innate response, which waned before the infection was controlled; this was followed by primarily adaptive immune responses that peaked at Week 5, which coincided with clearance of the infection. In conjunction with the latter, there was an increased expression of B cell-associated (Ig) genes at Week 6 that persisted through 11 weeks. In contrast, CD40L-KO mice, which are highly susceptible to developing severe Pneumocystis pneumonia, showed essentially no up-regulation of immune response-associated genes at Days 35-75. Immunohistochemical staining supported these observations by demonstrating an increase in CD4+, CD68+, and CD19+ cells in C57BL/6 but not CD40L-KO mice. Thus, the healthy host demonstrates a robust, biphasic response to infection by Pneumocystis; CD40L is an essential upstream regulator of the adaptive immune responses that efficiently control infection and prevent development of progressive pneumonia.

Pulmonary Interleukin-17-Positive Lymphocytes Increase during Pneumocystis murina Infection but Are Not Required for Clearance of Pneumocystis

“…These responses were temporally associated with the Pneumocystis organism load and peaked at approximately the same time as robust immune responses were detected by our earlier microarray studies (18). Of note, a higher proportion of cells produce IFN-␥ compared to IL-17, with the former accounting for ϳ25 to 30% of CD3 ϩ T cells compared to ϳ15% for the latter.…”

“…We initially characterized the frequency of NK cells, NKT cells, and ␥␦ T cells because our prior microarray studies in immunocompetent animals had suggested a potential role for these cells in early infection (maximum at ϳ14 days) (18). In three separate experiments, we analyzed these cell populations in animals that had been exposed for 7 to 24 days.…”

“…Given the importance of adaptive immunity in the clearance of Pneumocystis and our prior identification by microarray studies of a large number of genes related to adaptive immunity that showed increased expression that peaked at days 35 to 42 after exposure (18), we next focused on characterizing cell number and function during this period.…”

“…This cohousing infection model results in infection of animals with predictable kinetics; infection of healthy animals peaks at ca. 35 to 42 days and is cleared by ϳ70 days (18,44). For each experiment, one to two cages housed exposed mice, and one cage housed unexposed mice.…”

“…Among the genetic defects that have been studied to date in mouse models that do not lead to global loss of cell populations (e.g., Rag-deficient or scid mice), only the CD40-CD40L interaction has been shown to be absolutely required for ultimate control of Pneumocystis infection, since KO mice deficient in either CD40 or CD40L are highly susceptible to severe infection (18,32,42). Further studies are clearly needed to define the critical pathways required for immunity to Pneumocystis infection.…”