Autogenous humoral immunity to baboon xenotropic endogenous type C virus.

A radioimmune precipitation assay was used to evaluate the humoral immune response of baboons (Papio cynocephalus) to their endogenous, xenotropic, type C (M7) virus. Using 125I or [3H]leucine-labeled intact virus preparation, we demonstrated that baboons possess naturally occurring virus-binding antibodies to this virus. Antibody appeared directed against envelope and not internal viral proteins. Immunoglobulins responsible for virus-binding were predominant in the IgG and IgA serum fractions, whereas IgM participated to a minor degree. The biologic activities of these immunoglobulins were investigated by means of a focus reduction assay for virus neutralization with a murine sarcoma virus (M7) pseudotype. We determined that, despite the presence of considerable virus binding, none of the normal baboon sera tested possessed neutralizing activity for M7 virus.