The circumsporozoite protein of Plasmodium: a mechanism of immune evasion by the malaria parasite?

Sporozoites of malaria are covered with a repetitive surface antigen, the circumsporozoite (CS) protein. This antigen also appears to be a major target of the host immune response. The natural immunogenicity of the CS protein has led to attempts to develop the molecule as a vaccine candidate. It seems paradoxical, however, that a successful parasite should present to the host an immunogenic surface molecule which would induce protective immunity. In this paper we suggest that the CS protein is not the target of protective immunity under natural conditions, and that naturally immunogenic repetitive antigens in malaria and other parasites have evolved as a mechanism of immune evasion, via the induction of thymus-independent B-cell responses.