Potential for Early Diagnosis of Leptospirosis in Experimentally Infected Rhesus Macaques Based on Detection of Leptospiral Virulence-Modifying Protein Exotoxin Antigen and Antibody

Molecular and cellular mechanisms of leptospirosis pathogenesis remain poorly understood. Recent genomic, in vitro, and animal model data indicate that the novel PF07598 gene family encodes the so-called virulence-modifying proteins (VMPs), secreted exotoxins that are hypothesized to mediate the clinical manifestations of leptospirosis. Key to understanding the mechanistic role of VMPs in disease pathogenesis is testing the hypothesis that VMPs are produced during infection. Rhesus macaques were experimentally inoculated at multiple sites with low-passage Leptospira interrogans serovar Copenhageni strain Fiocruz L1-130. Microscopic agglutination test and polymerase chain reaction confirmed infection, but animals did not develop observable illness. An ELISA using recombinant VMPs demonstrated anti-VMP IgM and IgG antibodies on days 4 and 8, respectively, indicating that the infectious strain produced VMPs expectedly after infection. An antigen-capture ELISA detected VMP antigen in infected monkey urine and blood. Liver, kidneys, and lungs demonstrated mild histopathological abnormalities, demonstrating that target organs were subclinically involved in non-severe leptospirosis. Elevated serum levels of multiple cytokines (TNFa, IL-15, IL-8, sCD40L, MCP-1, and MIPb) showed systemic host inflammatory responses. This study addresses key gaps in understanding leptospirosis pathogenesis in nonhuman primates, aiming to develop VMP-based biomarkers for improved diagnosis and prognosis.