EVALUATION OF HUMORAL AND CELL-MEDIATED INDUCIBLE IMMUNITY TO HAEMOPHILUS-DUCREYI IN AN ANIMAL-MODEL OF CHANCROID

To study the mechanisms of inducible immunity to Haemophilus ducreyi i nfection in the temperature-dependent rabbit model of chancroid, we co nducted passive immunization experiments and characterized the inflamm atory infiltrate of chancroidal lesions, Polyclonal immunoglobulin G w as purified from immune sera raised against H. ducreyi 35000 whole-cel l lysate or a pilus preparation and from naive control rabbits, Rabbit s were passively immunized with 24 or 48 mg of purified polyclonal imm unoglobulin G intravenously, followed 24 h after infusion by homologou s titered infectious challenge, Despite titratable antibody, no signif icant difference in infection or disease was observed, We then evaluat ed the immunohistology of lesions produced by homologous-strain challe nge in sham-immunized rabbits and those protectively vaccinated by pil us preparation immunization, Immunohistochemical stains for CD5 and CD 4 T-lymphocyte markers were performed on lesion sections 4, 10, 15, an d 21 days from infection, Lesions of pilus preparation vaccinees compa red with those of controls had earlier infiltration with significantly more T lymphocytes (CD5(+)) and,vith a greater proportion of CD4(+) T lymphocytes at day 4 (33% +/- 5.5% versus 9.7% +/- 2%; P = 0.092), co rroborating earlier sterilization (5.0 +/- 2 versus 13.7 +/- 0.71 days ; P < 0.001) and lesion resolution, Intraepithelial challenge of pilus -vaccinated rabbits with 100 mu g of the pilus preparation alone produ ced indurated lesions within 48 h with lymphoid and plasmacytoid infil tration, edema, and extravasation of erythrocytes, We conclude that pa ssive immunization may not confer a vaccine effect in this model and t hat active vaccination with a pilus preparation induces a delayed-type hypersensitivity skin test response and confers protection through ce ll-mediated immunity seen as an amplified lymphocytic infiltrate and a ccelerated maturation of the T-lymphocyte response.