Antibody responses and protective immunity in rats receiving repeated inoculations of Strongyloides ratti.

The relationship between specific antibody responses and protective immunity against Strongyloides ratti was examined in rats receiving 10, 50, or 500 infective larvae (L3) at weekly intervals. No specific IgG response was detected in rats receiving 10-L3 inoculations for 7 wk. Fifty- and 500-L3 inoculations induced an IgG response by weeks 2 and 3, respectively, and a higher IgG response was induced in rats receiving the higher doses. All 3 inoculation doses induced high IgE responses, but the kinetics were different. IgE in the 10-L3 group continued to rise from weeks 4 to 7. In the 50- and 500-L3 groups, IgE was detected first at week 3 and increased until week 5. It then declined in the 500-L3 group and the titer at week 7 was significantly lower than that at week 5, whereas it remained the same in the 50-L3 group. The number of larvae recovered from the head 40 hr after a challenge inoculation (1,000 L3) significantly declined by weeks 7, 3, and 2 in rats receiving 10, 50-, and 500-L3 inoculations, respectively. Intestinal worm burdens increased for 7 wk in the 10-L3 group, 5 wk for the 50-L3 group, and 2 wk for the 500-L3 group. These findings indicate that repeated inoculations of low doses of L3 induce delayed and limited protective immunity to a heavy challenge and worm expulsion from the intestine. There was a temporal correlation between the levels of protection and serum IgG, whereas circulating IgE level did not seem to affect directly either the level of the resistance or expulsion of intestinal worms.