Humoral recall responses in HIV infection. Levels, specificity, and affinity of antigen-specific IgG

To evaluate the integrity of humoral immunologic memory among persons with HIV infection, we measured the levels, specificity, and functional affinity of circulating antibodies to vaccine-related recall Ag, tetanus (TT) and diphtheria toxoids (DT), and to naturally acquired measles virus, in sera from 17 HIV-seronegative control subjects, 17 asymptomatic HIV-seropositive patients, and 10 patients with AIDS. Preimmunization levels of TT- and measles-specific IgG were similar in all groups, although DT-specific IgG was lower in AIDS patients. Four wk after immunization with TT3 and DT, all groups showed significantly increased specific antibody levels (p less than 0.02). The asymptomatic HIV+ patients and control subjects achieved similar peak serum levels of TT-specific IgG (102 +/- 32 and 169 +/- 36 micrograms/ml, respectively). In contrast, the AIDS patients had lower peak values of both TT- and DT- specific IgG (p less than 0.05). Peak levels correlated directly with the number of CD4+ T cells (p less than 0.05). However, 80 to 100% of all participants tested, independent of HIV status, showed higher levels of TT- and DT-specific IgG 6 mo after immunization compared with preimmunization levels. The antitoxoid antibodies were specific as they did not cross-react with other Ag in competitive inhibition experiments. In addition, all groups exhibited antibodies to TT and DT both pre- and postimmunization of equivalent functional affinity (avidity) (Kd = 10(-10)-10(-11) mol/liter). We conclude that, in contrast to the profoundly depressed humoral responses to new Ag, persons with asymptomatic HIV infection retain humoral immunity to certain recall Ag. These levels of specific IgG to three recall Ag are not proportional to elevated levels of total serum IgG in HIV-infected patients. In addition, many patients with HIV respond to challenge with recall Ag by producing significant amounts of high affinity IgG that may persist over time.