Epitope

Protective cellular immune responses depend on MHC presentation of pathogen-derived Ag fragments. MHC diversity renders this process sensitive to point mutations coding for altered amino acid sequence of the short target Ag-derived peptides epitopes. Thus, in a given host, a pathogen with an altered epitope sequence will be more likely to escape detection and elimination by the immune system. At a population level, selection by immune pressure will increase the likelihood of polymorphism in important pathogen antigenic epitopes. This mechanism of immune evasion is found in viruses and other pathogens. The detection of polymorphic hot spots in an Ag is often taken as a strong indication of its role in protective immunity. We provide evidence that polymorphisms in the T cell epitopes of a malaria vaccine candidate are unlikely to have been selected by immune pressure in the human host. The circumsporozoite protein (CS)4, is a major component of the surface of the sporozoite. This is the form of a malaria parasite injected by the mosquito into the vertebrate host to initiate the malaria infection. CS has long been considered an Ag crucially implicated in protective immunity against the pre-erythrocytic (PE) stages of the Plasmodium infection . The gene coding for this Ag was the first Plasmodium gene to be cloned  and the first Plasmodium falciparum subunit vaccine tested in human volunteers was based on  CS-based vaccines are still at the forefront of malaria vaccine development