TRYPANOSOMA MUSCULI SURVIVAL IN THE KIDNEYS OF CHRONICALLY INFECTED MICE: KIDNEY FORM ULTRASTRUCTURE, SURFACE CHARACTERISTICS, AND SEROLOGICAL INTERACTIONS

After elimination of Trypanosoma musculi from the general circulation by the immune responses of infected mice, the animals are resistant to reinfection. Yet, parasites survive in the vasa recta of the kidneys for the life of these mice. These kidney forms (KF) actively reproduce in an environment that provides the necessary nutrients and appears to prevent their elimination from these capillaries by the hosts' immune responses. Comparative studies conducted with KF and the bloodstream forms (BSF) indicate that, although both forms appear to be similar morphologically at the ultrastructural level, they differ in their surface reactivities with lectins and tolerance to various pH and solute concentrations. Although antibodies are not detected on the surfaces of KF, urea levels approximating those in the vasa recta dissociate antibody from the surfaces of BSE The data suggest that parasites found in the vasa recta of these chronically infected mice differ from the BSF and are protected from the humoral and cell-mediated immune responses of the murine hosts by the concentrated solutes present in these capillaries. The KF may be killed by these same immune effector mechanisms upon leaving the capillaries of the kidneys and, therefore, not be found in the general circulation of these chronically infected immune hosts. Laboratory mice infected with Trypanosoma musculi display parasitemias, which terminate after approximately 3-4 wk. Bloodstream forms (BSF) are eliminated from the peripheral blood by the immune responses of the hosts, but parasites per- sist in the vasa recta of the kidneys for the lives of the mice (Viens et al., 1972). These mice with chronic or latent infec- tions are immune to reinfection and do not experience relapses, although isolated kidney forms (KF) and homogenates of kid- neys from these animals are infective when inoculated into na- ive mice (Monroy and Dusanic, 1997). Light microscopy sug- gests the KF and BSF are similar morphologically, and mea- surements of coefficients of variation indicate the KF are an actively dividing population within the vasa recta. Direct im- munofluorescent analyses do not detect antibodies on the KF surfaces, but indirect analyses with antisera from mice during the acute and chronic phases of the infections show that anti- bodies are capable of reacting with the parasites from the kid- neys (Monroy and Dusanic, 1997). These observations suggest that, whereas the KF are capable of initiating infections, they do not cause relapses in the resistant mice because of the es- tablished immunological responses. The vasa recta may be an immunologically privileged site for the parasites (Wilson, 1971), and KF that escape from the vasa recta are killed by the same responses that protect the host. Within these capillaries, they appear to be protected from the immune responses and multiply at a rate sufficient to maintain the chronic infection. Experiments were designed to further characterize the KF and gain some insight into the mechanisms responsible for their persistence in the kidneys of the chronically infected mice. The research included studies of the ultrastructural anatomy of the KF and BSF to determine whether any significant differences were apparent between the parasites. Lectins were used to ex- amine surface carbohydrates and as a sensitive test for com-