Cytokine and Chemokine Responses to Plasmodium Falciparum Malaria Vaccine Candidate Antigens During a Period of Interrupted Malaria Transmission in a Highland Area of Western Kenya

Abstract

Longitudinal studies of cytokine responses to Plasmodium Jalciparum (P. Jalciparum) antigens have depicted the acquisition and maintenance of clinical immunity, which developswith age. While the balance between cytokines has been shown to be critical in determiningthe outcome of clinical disease, cytokines have been implicated in altering haemoglobin (Hb) levels in areas of stable malaria transmission. However, to date, the stabilitiesof antigen-specific cytokine responses associated with protection, whether the responses are age-dependent, the balance between cytokine responses and their effect on Hb in an area of unstable malaria transmission following malaria interruption remains unknown, such as in western Kenya. Insight on the stabilities and balance of cytokine responsesto malaria vaccine candidates and their interaction with age and Hb in areas of low unstable malaria transmission would be beneficial as invaluable surrogate markers in evaluating vaccine immunogenicity and efficacy and in informing future vaccine development studies in similar populations. This longitudinal cohort study aimed at measuring the stabilities of antigen-specific cytokine response to multiple P. Jalciparum antigens by age, their effect on Hb concentrations and whether cytokine balance was affected during a period of interrupted malaria transmission. Peripheral blood mononuclear cells (PBMCs) were isolated from whole blood, cultured with apical membrane antigen (AMA)-l, circumsporozoite protein (CSP), liver stage antigen (LSA)-l, MB2, merozoite surface protein (MSP)-l and thrombospondin adhesive protein (TRAP) at 37°C and 5% CO2 for 5 days. Antigen-specific cytokine (interleukin (IL)-5, IL-6, IL-lO, interferon (IFN)-y and tumor necrosis factor (TNF)-a and chemokine [regulated upon activated, normal T-cells expressed and secreted (RANTES)]responses were assessed in 38 children and 62 adults from Nandi County in westernKenya, during a period of interrupted malaria transmission. McNemar's test was used to compare paired continuous variables, Wilcoxon signed-rank and Fisher's exact test to compare categorical variables, Wilcoxon rank sum test to compare continuous variables and Spearman's rank test to assess associations. Results revealed that levels of IL-6, and RANTES responses to almost all P. Jalciparum antigens were stable (P >0.05), while levels of IL-5, IL-IO IFN-y, and TNF-a to most P. Jalciparum antigens decreased . (P <0.05). No difference was observed between the antigen-specific cytokine responses by age (P>0.05). There were weak to moderate correlation within the cytokines (P<0.05), however, strong correlation between TNF-a and IFN-y response was observed (P < 0.05). Weak correlations were observed between cytokine (RANTES and TNF-a) levels and Hb levels (P <0.05). There was no difference in the TNF-a:IL-IO cytokine ratios by anemia status (P >0.05). For this population, antigen-specific IL-5, IFN-y, IL10, TNF-a decreased, while antigen-specific IL-6 and RANTES responses remained stable after a prolonged period of very low malaria transmission. RANTES and TNF-a weakly correlated with Hb levels. Furthermore, no age-related pattern in the antigenspecific cytokine responses exists. This study shows that P. Jalciparum-specific cytokine/chemokine responses require frequent boosting with antigens to be maintained in this area of unstable transmission. Vaccine development may consider formulating vaccines that could be administered at comparable dosage. Information on the intercytokine balance and their relationship with Hb may be used to assess those susceptible to severe disease. Future studies should investigate cellular sources of the cytokines to determine which were impaired by the malaria interruption. Further studies should also investigate antigen-specific IFN-y, TNF-a and IL- 10 responses as biomarkers of increasedpopulation-level susceptibility to malaria after prolonged lack of P. Jalciparum exposure.