Effects of maternal antibodies on protection and development of antibody responses to human rotavirus in gnotobiotic pigs

Abstract

Although maternal antibodies can protect against infectious disease in infancy, they can also suppress active immune responses. The effects of circulating maternal antibodies, with and without colostrum and milk antibodies, on passive protection and active immunity to human rotavirus (HRV) were examined in gnotobiotic pigs. Pigs received intraperitoneal injections of high-titer serum (immune pigs [groups 1 and 2]) from immunized sows, low-titer serum from naturally infected sows (control pigs [groups 3 and 4]), or no serum (group 5). Immune or control colostrum and milk were added to the diet of groups 2 and 4, respectively. After inoculation (3 to 5 days of age) and challenge (postinoculation day [PID] 21) with virulent HRV, the effects of maternal antibodies on protection (from diarrhea and virus shedding), and on active antibody responses (measured by quantitation of antibody-secreting cells [ASC] in intestinal and systemic lymphoid tissues by ELISPOT) were evaluated. Groups 1 and 2 had significantly less diarrhea and virus shedding after inoculation but higher rates of diarrhea and virus shedding after challenge than did groups 3 and 5. Group 1 and 2 pigs had significantly fewer immunoglobulin A (IgA) ASC in intestinal tissues at PID 21 and at postchallenge day (PCD) 7 compared to group 5. Significantly fewer IgG ASC were present in the intestines of group 2 pigs at PID 21 and PCD 7 compared to group 5. There was a trend towards fewer ASC in intestinal tissues of group 2 than group 1, from PID 21 on, with significantly fewer IgA ASC at PCD 7. IgG ASC in the duodenum and mesenteric lymph nodes of group 3 and 4 pigs were significantly fewer than in group 5 at PCD 7. These decreases in ASC emphasize the role of passive antibodies in impairing induction of ASC rather than in merely suppressing the function of differentiated B cells. To be successful, vaccines intended for populations with high titers of maternal antibodies (infants in developing countries) may require higher titers of virus, multiple doses, or improved delivery systems, such as the use of microencapsulation or immune stimulating complexes, to overcome the suppressive effects of maternal antibodies.

FIG. 1

(A) VN antibody titers in pools of sow serum, colostrum, and milk prepared from immunized sows (solid bars) and nonimmunized sows (open bars). (B) GMT of VN antibodies in serum of pigs at PID 0. Pigs received intraperitoneal injections of 60 ml of immune or control serum within 18 h of derivation. A subset of pigs received 15 ml of immune or control colostrum orally twice a day from 3 to 5 days of age and 25 ml of immune or control milk twice a day for another 7 days. Solid bars indicate titers in pigs inoculated with virulent Wa HRV at PID 0, and open bars indicate titers in pigs mock inoculated at PID 0. Columns labeled with an A differ significantly from those labeled with a B (one-way ANOVA of log-transformed VN titers followed by Duncan’s multiple-range test).

FIG. 2

GMT of VN antibodies in piglet serum from PID 0 to PID 28. Piglets were challenge exposed at PID 21 (PCD 0). Datum points labeled with different letters at the same time point differ significantly (one-way ANOVA of log-transformed VN titers followed by Duncan’s multiple-range test).

FIG. 3

Mean ASC counts per 5 × 10⁵ MNC in duodenum, ileum, and MLN. Symbols: ∗, group differs significantly from group 5 for the same isotype at the same time point (Kruskal-Wallis rank sum test); ▵, groups 1 and 2 differ significantly for the same isotype at the same time point (Kruskal-Wallis rank sum test); □, groups 3 and 4 differ significantly for the same isotype at the same time point (Kruskal-Wallis rank sum test).

FIG. 4

Mean ASC counts per 5 × 10⁵ MNC in spleen and peripheral blood. For an explanation of symbols, see the legend to Fig. 3. Note the change of vertical scale from Fig. 3.

FIG. 5

Mean IgSC counts per 5 × 10⁵ MNC in duodenum, ileum, and MLN. For statistical analysis, see Table 5. NA, not available.

FIG. 6

Mean IgSC counts per 5 × 10⁵ MNC in spleen and peripheral blood. For statistical analysis, see Table 5. NA, not available.