Indira S Pargass, William C Davis, K Jane Wardrop, Debby C Alperin, Stephen A Hines
Washington State University College of Veterinary Medicine

There is a distinct age associated susceptibility to Rhodococcus equi infection. Foals less than 6 months of age are typically affected but initial infection is thought to occur in the neonatal and perinatal period.  Affected foals develop life threatening pyogranulomatous bronchopneumonia. R. equi is closely related and structurally similar to Mycobacterium tuberculosis, the bacterium that causes tuberculosis.  Importantly, both organisms possess a cell wall containing unique lipids and glycoproteins.  Protective immune responses to M. tuberculosis involve classical MHC restricted CD4+ and CD8+ T cells that recognize peptide antigen, as well as MHC-independent T cells that recognize mycobacterial lipid antigen presented by CD1 molecules.  Most mammals examined this far possess at least one of the CD1 isoforms, although there are some striking species differences. Given the structural similarity between these two pathogens and our previous observations regarding R. equi-specific, MHC-unrestricted cyototoxic T lymphocytes (CTL), we developed 3 related hypotheses: (1) Antigen presenting CD1 molecules recognized by previously described monoclonal antibodies are expressed on equine antigen presenting cells (APC), (2) Perinatal equine APC express significantly lower levels of CD1 when compared to APC from adult horses, and (3) In response to R equi infection, APCs from both adult horses and perinatal foals upregulate CD1 expression.  Equine APCs were obtained from overnight culture of adult and perinatal foal peripheral blood monocytes.  These APCs had previously been shown to present R equi antigens to MHC-unrestricted CTL. Following infection with live virulent R. equi, CD1 expression was examined by flow cytometric analysis using a panel of CD1 antibodies with different species and isoform specificities. Four anti-CD1b antibodies cross reacted with both foal and adult APCs. Significant differences in CD1 expression were not observed between adult and perinatal foals. CD1 expression also appeared to be unaffected by R. equi infection. The cross reactivity observed suggests that equine CD1b molecule is evolutionarily conserved. This correlates with the finding that CD1 molecules are conserved among mammalian species. The findings also suggest that R. equi infection does not alter CD1 expression, and that the susceptibility of foals to rhodococcal pneumonia is not related to decreased CD1 expression compared with adults. If in fact CD1 does contribute to cell mediated immunity against R equi, these observations suggest that the age related susceptibility is not due to impaired CD1 expression during the susceptible perinatal period.