Depletion of CD8+ cells abolishes memory in acquired immunity against Chlamydia pneumoniae in BALB/c mice

The importance of T cells in Chlamydia pneumoniae infection in mice was assessed by comparing wild‐type BALB/c mice with nude mice and mice depleted in vivo of either CD4+ or CD8+ T cells. Whereas wild‐type mice cleared the primary infection in 3 weeks, nude mice were only able to restrict the infection and could not clear it during the observation period of 56 days. Nude mice exhibited a greater number of macrophages in their lungs and the pulmonary cells secreted a higher level of tumour necrosis factor‐α (TNF‐α) than wild‐type mice. Depletion of CD4+ cells did not change the overall infection kinetics of the primary infection. However, depletion of CD8+ cells resulted in a slightly impaired clearance of the bacteria in the late stages of primary infection. To assess the role of the two T‐cell subsets in the acquired immunity that develops during primary infection in wild‐type BALB/c mice, in vivo depletions were performed during reinfection. Prior to reinfection, immunocompetent wild‐type mice were infected and natural immunity was allowed to form. During reinfection, depletion of CD4+ cells did not have any effect on infection kinetics, whereas depletion of CD8+ cells abolished the protection, reverting the infection kinetics and bacterial load to the same levels found in wild‐type mice during primary infection. These results show that T cells are necessary for clearing C. pneumoniae infection in mice. Furthermore, whereas neither of the two main T‐cell subsets, separately, were essential for clearance of primary infection, the induced protective immunity was strongly CD8 dependent.