Incomplete dominance of the low antibody response to Cryptosporidium parvum antigens in mice selected for high and low antibody responsiveness

The humoral antibody response to Cryptosporidium was investigated in mice genetically selected for high (H) and low (L) antibody responsiveness. Groups of 4-5 mice from two different selections, general primary (GP) and general secondary (GS), were studied. Following immunization with Cryptosporidium parvum antigens, the maximum levels of IgG in the H_GP (X̄ ± SD = 1.13 ± 0.35, N = 5) and in the H_GS (0.42 ± 0.15, N = 4) lines, and of IgM in the H_GP line (0.86 ± 0.53, N = 5) were significantly higher than those in their L counterparts (0.04 ± 0.02, N = 5; 0.05 ± 0.02, N = 4 and 0.24 ± 0.07, N = 5, respectively). These findings were similar to those reported for other immunogens. However, the IgG (0.22 ± 0.05, N = 4) and the IgM (0.33 ± 0.08, N = 4) responses to immunization of F¹ (L_GP × H_GP) hybrids indicated an incomplete dominance of the low response, in contrast to the incomplete dominance of the high response described for many other antigens and representing an important exception. In addition, the H, L and F₁ mice did not develop detectable infections when inoculated with live Cryptosporidium oocysts, supporting the view that a reduced or zero antibody production itself is not enough to permit the establishment of Cryptosporidium infection in adult mice.