The specific contribution of interleukin-17/interleukin-17 receptor (IL-17/IL-17R)-mediated responses in regulating host susceptibility against obligatory intracellular Chlamydia infection was investigated in C57BL/6 and C3H/HeN mice during Chlamydia muridarum respiratory infection. We demonstrated that Chlamydia stimulated IL-17/IL-17R-associated responses in both Chlamydia-resistant C57BL/6 and Chlamydia-susceptible C3H/HeN mice. However, C3H/HeN mice developed a significantly greater IL-17/IL-17R-associated response than C57BL/6 mice did. This was reflected by an increase in IL-17 mRNA expression, a higher recall IL-17 production from splenocytes upon antigen restimulation, and higher production of Th17-related cytokines (IL-23 and IL-6) and chemokines (chemokine [C-X-C motif] ligand 2 [CXCL1]/keratinocyte-derived chemokine [KC] and CXCL2/macrophage inflammatory protein 1 [MIP2]) in C3H/HeN mice than in C57BL/6 mice. Furthermore, C3H/HeN mice displayed a massive accumulation of activated and preactivated neutrophils in the airway and lung parenchyma compared to their C57BL/6 counterparts. We further demonstrated that the skewed IL-17/Th17 profile in C3H/HeN mice was predisposed by a higher basal level of IL-17 receptor C (IL-17RC) expression and then further amplified by a higher inducible IL-17RA expression in lungs. Most importantly, in vivo delivery of IL-17RA antagonist that resulted in a 50% reduction in the neutrophilic infiltration in lungs was able to reverse the susceptible phenotype of C3H/HeN mice to respiratory Chlamydia infection. Thus, our data for the first time have demonstrated a critical role for the IL-17/IL-17R axis in regulating host susceptibility to Chlamydia infection in mice.
