TLR4 competence and mouse models of leptospirosis

Mice are slowly being accepted as alternative models for investigation of leptospiral infection. The strain most used to analyse sublethal disease (C3H-HeJ) expresses a tlr4 gene in its immune cells that is hyporesponsive to leptospiral LPS and thus the model is deemed immunocompromised. To help resolve this valid scientific concern we did a study in which we compared infection with Leptospira interrogans serovar Copenhageni Fiocruz in mice expressing fully competent tlr4 (C3H-HeN, C57BL6) versus tlr4 hyporesponsive mice (C3H- HeJ) over a period of two weeks. We found that the C3H-HeJ mouse was the only strain that sustained lethal infection, thus confirming the association between tlr4 function and acute disease. We found that the two mouse strains with a fully functional tlr4 gene (C3H-HeN and C57BL/6) also developed clinical and molecular signs of sublethal leptospirosis nearly on par with C3H-HeJ, as quantified by weight loss, survival curves, presence of Leptospira 16S rRNA in blood and urine, and burden of viable spirochetes in kidney. Regarding kidney markers of inflammation and fibrosis, C3H-HeN and C57BL/6 produced less IL-1 beta, iNOS and ColA1 than C3H-HeJ, which is consistent with increased resilience to infection expected of the tlr4 competent strains. Regarding Leptospira-specific antibody, all mouse strains produced IgM and T-cell independent IgG3. Interestingly, in contrast to C57BL/6, both C3H strains produced circulating IgG1 but only C3H-HeN produced IgG2a to L. interrogans two weeks post infection. Thus, TLR4-competent C3H-HeN and C57BL/6 may be more appropriate mouse models of sublethal leptospirosis, whereas C3H-HeJ can be used to further establish a mouse model of lethal leptospirosis. Our data strongly suggests that TLR4 function is important but not sufficient to cause susceptibility to leptospirosis.