Differences in the immune response against ruminant chlamydial strains in a murine model

CBA/J mice were used in the present study to establish differences between the immune response to three chlamydial strains: AB7 (Chlamydia psittaci wild-type strain), 1B (C. psittaci vaccinal strain) and iB1 (C. pecorum). The evolution of chlamydial infection was evaluated in each strain by studying the clinical signs, the number of bacteria isolated from the spleen and the pathology of the liver. Three aspects of the immune response were then studied: the characterization of the infiltrate of leukocytes in the liver, the percentages of T- and B-cells, macrophages and neutrophils in the spleen, and the presence of cytokines in the serum. Infection followed a different course in the C. psittaci-infected mice; 1B-infected mice showed milder levels in all the parameters analysed than their AB7-infected counterparts. The resolution of infection was earlier in 1B-infected mice and, although the immune response to both strains was Th1-like, a more intense CD8+ T-cell response and an earlier presence of TNF-alpha in serum were observed in this group. C. pecorum infection was controlled mainly by a non-specific immune response, since these mice showed no signs of a systemic specific immune response. Neutrophil depletion experiments showed that these cells play a very limited role in the non-specific response against C. pecorum.