ASSOCIATION OF INTERFERON-γ RESPONSES TO PRE-ERYTHROCYTIC STAGE VACCINE CANDIDATE ANTIGENS OF PLASMODIUM FALCIPARUM IN YOUNG KENYAN CHILDREN WITH IMPROVED HEMOGLOBIN LEVELS: XV. ASEMBO BAY COHORT PROJECT

Abstract

Abstract. Previous studies in animal models have revealed an association between interferon-␥ (IFN-␥), produced by CD8 + T cells and irradiated sporozoite-induced sterile immunity. To determine whether IFN-␥ can serve as a marker of pre-erythrocytic protective immunity in individuals naturally exposed to malaria, we characterized IFN-␥ and lymphocyte proliferative responses to previously defined CD8 + cytotoxic T lymphocyte (CTL) epitopes from six preerythrocytic stage antigens in 107 children six months to two years old from a community-based birth cohort in western Kenya. We found that IFN-␥ positive responders had higher hemoglobin (Hb) levels and significantly reduced prevalence of severe malarial anemia one month after the test compared with IFN-␥ non-responders, suggesting that IFN-␥ immune responses to these pre-erythrocytic antigens were associated with protection against malarial anemia. Children who responded by lymphocyte proliferation had a significantly longer time to first documented malaria parasitemia after birth; however, there was no correlation between the presence of lymphocyte proliferative response and higher Hb levels. We propose that IFN-␥ production could be used as a potential marker of protective immunity against malaria associated anemia in young children living in malaria holoendemic areas.

Gamma Interferon Responses toPlasmodium falciparumLiver-Stage Antigen 1 and Thrombospondin-Related Adhesive Protein and Their Relationship to Age, Transmission Intensity, and Protection against Malaria

“…In the present study, frequencies of IFN-␥ responses to LSA-1 were greater by ELISA than by ELISPOT in the lowland, stable-transmission area. The PBMC incubation period for IFN-␥ testing by ELISA and ELISPOT used in the present study was similar to that used in prior studies documenting an association of protection from infection or disease with ELISA IFN-␥ responses to preerythrocytic antigens (27,35) but longer than that used in studies assessing ex vivo ELISPOT IFN-␥ responses to TRAP and CSP (11,37). The longer incubation period was used for ELISPOT because ELISPOT IFN-␥ responses were not found in this study population after the shorter 16-h incubation period.…”

“…IFN-␥ responses to LSA-1, for example, were reported to be associated with protection against subsequent infection in children with mild malaria in Gabon (27) but did not correlate with time to infection after drug-induced parasitologic cure of adults and children in Kenya (20,25). Similarly, preexisting IFN-␥ responses to TRAP correlated with higher hemoglobin levels in a prospective examination of Kenyan children (35), whereas there was not a significant relationship between IFN-␥ response and attacks of clinical malaria in children and adults in another area of Kenya (11). The apparently conflicting conclusions of such studies may stem from technical, demographic, and epidemiologic factors.…”

“…This issue has been addressed in field-based studies conducted in several areas of the world where malaria is endemic, including sub-Saharan Africa, Papua New Guinea, and Latin America. These reports in general show that: (i) peptide-stimulated IFN-␥ may be produced by both CD4 ϩ and CD8 ϩ T cells (3,6,11,12,20); (ii) responses to LSA-1 and TRAP peptides, as measured by various methods and with various numbers of peptides, are demonstrable in ϳ10 to 40% of individuals, with studies of LSA-1 (3, 6, 20) but not TRAP (11) demonstrating an age-related increase in the rate of responses; (iii) the instability in ELISPOT IFN-␥ responses exists at the individual level (11); and (iv) an inconsistent relationship exists between these responses and parasitologic and clinical endpoints such as time to reinfection and malaria morbidity (11,20,25,27,35).…”

“…From a technical perspective, the method of assessing IFN-␥ production may be important. Recent studies conducted in The Gambia of responses to TRAP and circumsporozoite protein (CSP) assessed the number of IFN-␥-secreting mononuclear cells by the enzyme-linked immunospot assay (ELISPOT) (10,11,37), while most studies of LSA-1 have measured IFN-␥ production by enzyme-linked immunosorbent assay (ELISA) (20,25,27,35). Interestingly, although an early report suggested an increased sensitivity of the ELISPOT compared to the ELISA for detecting T-cell IFN-␥ responses in persons naturally exposed to malaria (23), there have been to our knowledge no subsequent comparative studies of the two methods for residents of areas where malaria is endemic.…”

“…Interestingly, although an early report suggested an increased sensitivity of the ELISPOT compared to the ELISA for detecting T-cell IFN-␥ responses in persons naturally exposed to malaria (23), there have been to our knowledge no subsequent comparative studies of the two methods for residents of areas where malaria is endemic. Furthermore, all published studies that have reported an association of LSA-1-or TRAP-induced IFN-␥ responses with protection against malaria infection or disease have assessed IFN-␥ production by ELISA (6,27,29,35). With respect to demographic and epidemiologic variables, the seasonality, stability, and intensity of malaria transmission; the ages and genetic backgrounds of the study participants; and the specific clinical and parasitologic variables chosen as endpoints may influence the correlation with preexisting or concurrent cytokine responses.…”