Oral immunization with recombinant listeria monocytogenes controls virus load after vaginal challenge with feline immunodeficiency virus

Abstract

Recombinant Listeria monocytogenes has many attractive characteristics as a vaccine vector against human immunodeficiency virus (HIV). Wild-type and attenuated Listeria strains expressing HIV Gag have been shown to induce long-lived mucosal and systemic T-cell responses in mice. Using the feline immunodeficiency virus (FIV) model of HIV we evaluated recombinant L. monocytogenes in a challenge system. Five cats were immunized with recombinant L. monocytogenes that expresses the FIV Gag and delivers an FIV Env-expressing DNA vaccine (LMgag/pND14-Lc-env). Control cats were either sham immunized or immunized with wild-type L. monocytogenes (LM-wt). At 1 year after vaginal challenge, provirus could not be detected in any of the nine tissues evaluated from cats immunized with the recombinant bacteria but was detected in at least one tissue in 8 of 10 control animals. Virus was isolated from bone marrow of four of five LMgag/pND14-Lc-env-immunized cats by use of a stringent coculture system but required CD8(+) T-cell depletion, indicating CD8(+) T-cell suppression of virus replication. Control animals had an inverted CD4:CD8 ratio in mesenteric lymph node and were depleted of both CD4(+) and CD8(+) intestinal epithelial T cells, while LMgag/pND14-Lc-env-immunized animals showed no such abnormalities. Vaginal FIV-specific immunoglobulin A was present at high titer in three LMgag/pND14-Lc-env-immunized cats before challenge and in all five at 1 year postchallenge. This study demonstrates that recombinant L. monocytogenes conferred some control of viral load after vaginal challenge with FIV.

FIG. 1.

Lymphocyte subsets in mesenteric lymph node. CD4:CD8 ratios (A) and percentages of CD4⁺ (B) and CD8⁺ (C) T cells are shown in a box (enclosing 50% of the data with the median value displayed as a line) and whisker (lines extending from the bottom and top of each box mark the minimum and maximum values) plot. An outlier is displayed as an individual point in panel B. The mean CD4:CD8 ratio for LMgag/pND14-Lc-env (LM-gag/env)-immunized group is significantly greater than that for the LM-wt- or sham-immunized group (P = 0.04 [ANOVA]).

FIG. 1.

Lymphocyte subsets in mesenteric lymph node. CD4:CD8 ratios (A) and percentages of CD4⁺ (B) and CD8⁺ (C) T cells are shown in a box (enclosing 50% of the data with the median value displayed as a line) and whisker (lines extending from the bottom and top of each box mark the minimum and maximum values) plot. An outlier is displayed as an individual point in panel B. The mean CD4:CD8 ratio for LMgag/pND14-Lc-env (LM-gag/env)-immunized group is significantly greater than that for the LM-wt- or sham-immunized group (P = 0.04 [ANOVA]).

FIG. 2.

Lymphocyte subsets in intestinal epithelium. Percentages of CD4⁺ (A) and CD8⁺ (B) T cells are shown for three FIV-naïve, SPF control cats, three sham-immunized cats, and three LMgag/pND14-Lc-env (LM-gag/env)-immunized cats.

FIG. 3.

Induction of FIV anti-p24 IgA in vaginal secretions. FIV Gag-specific IgA levels were measured in vaginal washes from sham-immunized (A), LM-wt-immunized (B), and LMgag/pND14-Lc-env-immunized (C) cats by ELISA at 0, 4, 8, 16, 20, 24, and 60 weeks postimmunization. Cats were vaginally challenged with FIV at week 12. Titers were determined by endpoint dilution.

FIG. 3.

Induction of FIV anti-p24 IgA in vaginal secretions. FIV Gag-specific IgA levels were measured in vaginal washes from sham-immunized (A), LM-wt-immunized (B), and LMgag/pND14-Lc-env-immunized (C) cats by ELISA at 0, 4, 8, 16, 20, 24, and 60 weeks postimmunization. Cats were vaginally challenged with FIV at week 12. Titers were determined by endpoint dilution.