Quantitative transmission of Dipetalonema viteae larvae into jirds, Meriones unguiculatus, and subsequent development of the larvae has not been studied. An attempt was made here to study the effects of size of inoculum (5, 20, 75 or trickle inoculation of 75 larvae) on number and size of adult worms recovered, the prepatent period, and the immunological response of the jirds. The study also focused on immunoregulatory activity the parasite may induce in a naive host which may enhance survival. Results show that patent infection may be established in jirds with as few as five infective larvae. There was no difference in length of prepatent period among animals given the various inocula. Adult worms recovered at autopsy showed an inverse correlation between the number of larvae inoculated with the length of adult worms. These studies suggest further a positive correlation between the size of inoculum and level of microfilaremia. Determination of the activity of macrophages, as measured by assessment of lysozyme content, also showed a correlation between size of inoculum, death of larvae, calcification, and macrophage activation. Peripheral blood eosinophilia was monitored in both normal and infected jirds. All infected jirds showed eosinophilia irrespective of size of inoculum. Studies on the immunological response to various levels of infection showed a dichotomy of response to mitogen and filarial antigen. While the response of lymph node cells from infected jirds was comparable to that of normal jird cells, the spleen cells of infected jirds showed suppressed responsiveness. Further investigation showed an inverse correlation between the responsiveness of spleen cells from infected jirds to mitogens and to filarial-derived antigen. The in vitro lymphocyte blastogenic response to filarial antigens was stage-specific and varied over the course of the infection. Analysis of the immunoregulatory mechanisms responsible for cell immunosuppression observed revealed a nylon wool, plastic-adherent cell to be involved. In vivo responsiveness to a heterologous antigen, sheep red blood cells, was also suppressed. The antibody response to adult worm antigen was not altered and showed an early sharp rise in titer which gradually plateaued
