OMP Gene Regulation. Genome sequencing studies indicate that pathogenic Leptospira species have a rich diversity of signal transduction mechanisms (6). Consistent with this, we have shown that osmolarity and temperature conditions found in host tissues rapidly and dramatically induce expression of LigA, LigB, and other leptospiral OMPs (7). These data indicate that OMPs upregulated in vivo play key roles in leptospiral colonization of host tissues at portals of entry during acquisition and transmission of infection. Surface-exposed OMPs that are upregulated during infection and mediate host tissue interactions are also important targets of a protective immune response. We, and others, have shown that LigA is a potent protective immunogen (8) and we have now localized the key LigA protective epitopes to domains 10-13 (9