C3H-HeN as a TLR4 competent mouse model of sublethal leptospirosis

AbstractMice 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. Author summaryPrevious studies by different laboratories have shown C3H-HeJ to be a susceptible mouse strain to sublethal and lethal leptospirosis. The goal of our study was to evaluate if low tlr4 responsiveness to leptospiral LPS in C3H-HeJ mice is the factor that drives susceptibility to infection with pathogenic Leptospira. We did a comparative study using mouse strains immunocompetent and hyporesponsive to tlr4. The data shows that although tlr4 does affect the virulence of leptospirosis (only C3H-HeJ succumbed to infection), the tlr4 competent strains (C3H-HeN and C57BL/6) also developed measurable sublethal leptospirosis. Thus, competent recognition of leptospiral-LPS by murine TLR4 plays a role but it is not sufficient to determine susceptibility to leptospirosis. Furthermore, the data suggests that T-cell dependent responses could be involved in susceptibility to leptospirosis in the C3H models of infection.