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Low CD8+ T lymphocyte response to P. falciparum circumsporozoite protein in naturally-exposed malaria endemic populations in Thailand.

Cytotoxic T lymphocytes (CTLs) specific for epitope(s) within the circumsporozoite (CS) protein of malaria sporozoite have been shown to play an important role in protective immunity against malaria. Human CTLs against the potential epitope at the carboxy terminal region of CS protein of Plasmodium falciparum 7G8 strain (Pf7G8CS 368-390) were determined in thirty-six falciparum malaria patients and ten healthy controls. Four of 36 individuals and none of the healthy controls developed Pf7G8CS 368-390 specific CTL activity. The CTL activity was antigen specific and CD8+ T cell dependent. Although low CTL response has been determined, the study suggested that there was a correlation between initial parasitemia and the specific Pf7G8CS 368-390 CTL activity. A correlation between such CTL activity and anti-R32tet32 antibody levels among individuals with previous malaria experience was found, which was in contrast to those among individuals with recent malaria infection. All these 4 CTL positive individuals had at least two episodes of clinical malaria experience while all 25 individuals who were exposed to malaria for the first time did not have such a specific CTL response. These results showed that individuals with a history of natural endemic exposure to P. falciparum sporozoite developed low specific CTL responses to Pf7G8CS 368-390, so that previous but recent sporozoite exposure might be a prerequisite for generation of such CS protein specific CTL response.