Low levels of IgG antibodies against pneumocystis carinii among HIV-infected patients

IgG antibodies against Pneumocystis carinii (P. carinii) were detected by an ELISA method using urea-extracted material from human and rat P. carinii as the antigen. Carbohydrate formed a major part of the antigen responsible for reactivity in the ELISA assay, since periodate treatment reduced the reactivity of most sera tested. Cross-reactivity between human and rat P. carinii was detected. However, human serum recognized antigens specific for human P. carinii. With the ELISA method IgG antibody levels were compared between blood donors (n = 40), asymptomatic HIV-antibody positive patients (n = 30) and AIDS patients with (n=22) and without previous P. carinii pneumonia (PCP) (n=21). HIV-infected patients had significantly lower antibody reactivity against the microorganism compared with blood donors. Among HIV-antibody positive patients the highest antibody reactivity was seen in PCP patients. The antibody response to PCP was impaired, since an equal number of patients had an increase and a decrease in antibody reactivity. In conclusion, carbohydrate formed an important part of the P. carinii immunogenic antigen. Cross-reactivity between rat and human P. carinii was demonstrated, but reactivity was somewhat lower using antigen from rats. The antibody level was lower in HIV-infected patients and the ability to mount an antibody response to the infection was impaired, suggesting that the poor antibody response may contribute to the liability of HIV-infected patients to have PCP.