Experimental neonatal syphilis. II. Immunological responses of neonatal rabbits to intradermal inoculation with Treponema pallidum (Nichols strain).

The immunological competence of neonatal rabbits inoculated intradermally with Treponema pallidum was examined. Both cellular responses and the production of humoral antibody to specific T. pallidum antigens and to nonspecific antigens or mitogens were investigated. In blast transformation assays, splenic and popliteal lymph node lymphocytes from neonates inoculated with virulent T. pallidum responded to T. pallidum antigens in a manner similar to or greater than inoculated adult rabbits. Splenic and popliteal lymph node lymphocytes from both uninoculated and T. pallidum-inoculated neonate and adult animals showed consistent and similar responses to concanavalin A. Both neonate and adult animals inoculated with heat-killed T. pallidum also responded but to a significantly lesser degree. Immunofluorescent examination of skin sections from the site of inoculation of adult and neonatal animals revealed 1) that the early infiltrate was composed predominantly of T cells, 2) diffuse antibody staining with rare B cells, and 3) fewer treponemes with significant fragmentation in neonates as compared to adult controls. Antibody production by neonates inoculated with virulent T. pallidum was delayed 4 to 6 weeks postinoculation as measured by the fluorescent Treponemal antibody absorption and Venereal Disease Research Laboratory (VDRL) test procedures, respectively. Antibody was not detected among neonates inoculated with heat-killed treponemes during a 6-week observation period and only low levels of VDRL antibody were detected in a few adult control animals. Evidence for incomplete resistance of neonatal rabbits to the intradermal inoculation of Treponema pallidum was provided.(ABSTRACT TRUNCATED AT 250 WORDS)