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Abstract Background Naturally acquired immunity to blood-stage Plasmodium falciparum infection develops with age and after repeated infections. In order to identify immune surrogates that can inform vaccine trials conducted in malaria endemic populations and to better understand the basis of naturally acquired immunity it is important to appreciate the temporal stability of cellular and humoral immune responses to malaria antigens. Methods Blood samples from 16 adults living in a malaria holoendemic region of western Kenya were obtained at six time points over the course of 9 months. T cell immunity to the 42 kDa C-terminal fragment of Merozoite Surface Protein-1 (MSP-142) was determined by IFN-γ ELISPOT. Antibodies to the 42 kDa and 19 kDa C-terminal fragments of MSP-1 were determined by serology and by functional assays that measure MSP-119 invasion inhibition antibodies (IIA) to the E-TSR (3D7) allele and growth inhibitory activity (GIA). The haplotype of MSP-119 alleles circulating in the population was determined by PCR. The kappa test of agreement was used to determine stability of immunity over the specified time intervals of 3 weeks, 6 weeks, 6 months, and 9 months. Results MSP-1 IgG antibodies determined by serology were most consistent over time, followed by MSP-1 specific T cell IFN-γ responses and GIA. MSP-119 IIA showed the least stability over time. However, the level of MSP-119 specific IIA correlated with relatively higher rainfall and higher prevalence of P. falciparum infection with the MSP-119 E-TSR haplotype. Conclusion Variation in the stability of cellular and humoral immune responses to P. falciparum blood stage antigens needs to be considered when interpreting the significance of these measurements as immune endpoints in residents of malaria endemic regions.

Abstract Background In areas of high-level, year-round malaria transmission, morbidity and mortality due to malaria decrease after the first two to three years of life. This reduction may be related to the development of cellular immunity to specific antigens expressed in the different life-cycle stages of Plasmodium falciparum. Methods A cross sectional study was conducted to evaluate T cell cytokine responses to the P. falciparum pre-erythrocytic antigen liver-stage antigen-1 (LSA-1) and the blood-stage antigen merozoite-surface protein-1 (MSP-1) in children under five years of age residing in a malaria holoendemic region of western Kenya. Interferon-γ (IFN-γ) and interleukin-10 (IL-10) responses to the LSA-1 T3 peptide (aa 1813–1835) and the MSP-1 aa20–39 peptide were tested in 48 children. Results The proportion of children producing IFN-γ to LSA-1 and to MSP-1 increased with age: in the 0–12, 13–24, 25–36 and 37–48 month age groups, zero, 11.1, 36.4 and 40% of children had IFN-γ responses to LSA-1 (p = 0.019), and 10, 10, 27.7 and 40% of children had IFN-γ responses to MSP-1 (p = 0.07), respectively. In contrast, the proportion of children producing IL-10 to LSA-1 and MSP-1 was similar in all age groups. Conclusion The data suggest that development of IFN-γ responses to LSA-1 and MSP-1 requires increased age and/or repeated exposure, whereas IL-10 responses to these antigens may occur at any age and with limited exposure. The data also demonstrate that by the age of 4 years, children in a malaria holoendemic area develop frequencies of IFN-γ responses to LSA-1 and MSP-1 similar to those seen in adults in the area.

Coinfection with Plasmodium falciparum malaria and Epstein-Barr virus (EBV) is a major risk factor for endemic Burkitt lymphoma (eBL), still one of the most prevalent pediatric cancers in equatorial Africa. Although malaria infection has been associated with immunosuppression, the precise mechanisms that contribute to EBV-associated lymphomagenesis remain unclear. In this study, we used polychromatic flow cytometry to characterize CD8(+) T-cell subsets specific for EBV-derived lytic (BMFL1 and BRLF1) and latent (LMP1, LMP2, and EBNA3C) antigens in individuals with divergent malaria exposure. No malaria-associated differences in EBV-specific CD8(+) T-cell frequencies were observed. However, based on a multidimensional analysis of CD45RO, CD27, CCR7, CD127, CD57, and PD-1 expression, we found that individuals living in regions with intense and perennial (holoendemic) malaria transmission harbored more differentiated EBV-specific CD8(+) T-cell populations that contained fewer central memory cells than individuals living in regions with little or no (hypoendemic) malaria. This profile shift was most marked for EBV-specific CD8(+) T-cell populations that targeted latent antigens. Importantly, malaria exposure did not skew the phenotypic properties of either cytomegalovirus (CMV)-specific CD8(+) T cells or the global CD8(+) memory T-cell pool. These observations define a malaria-associated aberration localized to the EBV-specific CD8(+) T-cell compartment that illuminates the etiology of eBL.

Endemic Burkitt lymphoma (eBL) is the most common childhood cancer in equatorial Africa and is linked to Epstein-Barr virus (EBV) and Plasmodium falciparum coinfections early in life. Epstein-Barr nuclear antigen 1 (EBNA1) is the sole viral latent antigen expressed in BL tumors. Loss of EBNA1-specific immune surveillance could allow eBL emergence. Therefore, EBNA1-specific T cell responses were analyzed by IFN-gamma ELISPOT in Kenyan children with eBL and compared to healthy children with divergent malaria exposure. Significantly fewer children with eBL, 16% (7/44) had EBNA1-specific IFN-gamma responses in contrast to healthy children living in a malaria holoendemic area or in an area with sporadic malaria transmission, 67% (40/60) and 72% (43/60) responders, respectively (p < 0.003). Children with eBL maintained IgG(1) dominated antibody responses to EBNA1 similar to healthy children suggesting a selective loss of IFN-gamma secreting EBNA1-specific T cells in the presence of intact humoral immunity. CD8(+) T cell responses to EBV lytic and latent antigens not expressed in the tumors were similarly robust in eBL patients compared to healthy children. In addition, CD4(+) T cell responses to a malaria protein, merozoite surface protein 1, were present in lymphoma patients. This study demonstrates a selective loss of EBNA1-specific T cell responses in children with eBL and suggests a potential immunotherapeutic target for this EBV-associated lymphoma.