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

Abstract

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-alpha (TNF-alpha) 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.

Figure 1

Chlamydia pneumoniae was cultured from the supernatants of homogenized lung samples of wild-type BALB/c mice and athymic nude mice after primary challenge with 10⁶–10⁷ inclusion-forming units (IFUs) of C. pneumoniae given intranasally in a volume of 40 μl. Data represents the mean logarithmic values obtained from individual mice. Fifteen to 33 wild-type mice and 17–30 nude mice were used per time-point. The arrow shows the detection limit of the culture assay: 1·3 IFU/lung. *Statistically significant difference between nude and wild-type mice (P = 0·001, P = 0·001, P < 0·01, P < 0·001) as determined by the Mann–Whitney U-test.

Figure 2

The numbers of cultured Chlamydia pneumoniae inclusion-forming units (IFUs) from the lungs of wild-type and in vivo CD8- or CD4-depleted BALB/c mice during primary infection and reinfection. Data represents the mean logarithmic values obtained from individual mice. The data for wild-type mice are the same as detailed in the legend to Figure 1. Numbers of mice used in the depletion experiments are shown in boxes inside the bars. The detection limit of the culture assay, 1·3 IFU/lung, is shown by an arrow. *Statistically significant difference compared with wild-type mice (P < 0·01, P < 0·01 after primary challenge and P = 0·001, P < 0·001, P < 0·05 after rechallenge). ND, not determined.

Figure 3

Induction of (a) proliferation and (b) secretion of tumour necrosis factor-α (TNF-α), (c) interferon-γ (IFN-γ) and (d) interleukin-10 (IL-10) by pulmonary cells, isolated on the indicated days after primary Chlamydia pneumoniae infection from wild-type, CD8-depleted and nude mice, in response to in vitro stimulation with inactivated C. pneumoniae. In the late stage of infection (days 18–33), data from typically two different time-points are combined. In panel (d) the scale is one-tenth that of panels (b) and (c). The proliferation index was calculated as described in the Materials and methods. Cytokine secretion results are expressed as C. pneumoniae-induced cytokine secretion after subtraction of the background secretion. BD, below the detection limit; ND, not determined.