Quantitative Assessment of T Cell Repertoire during Plasmodium falciparum Infection

Abstract

Malaria pathogenesis is an intricate process mediated by factors found in both the host and the parasite. The adaptive immune response to the most lethal malaria parasite, Plasmodium falciparum, remains to be fully comprehended. Meanwhile, T cells, which are critical for the host responses interact with antigens coupled withMHC molecules, through their membrane surface-expressed T cell receptor (TCR). These TCRs are produced by a process of imprecise gene rearrangement called V(D)J recombination, which is critical for antigen recognition. This may indicate that, the interaction between lymphocytes and pathogens alters the lymphocytes' frequencies and, hence, diversity of their antigen receptor repertoire, suggesting that both the repertoire and the diversity of the antigen receptors may differ at various disease stages. Therefore, the dynamics of T cells in children during Plasmodium falciparum infections were examined using peripheral blood mononuclear cells from children residing in a hyperendemic area in Ghana. The frequency of activated Tregs and T cell activation markers during P. falciparum infections, and the relationship between these markers and parasitaemia in children with acute P. falciparum infections were investigated. In addition, the expression profile of inhibitory andimmunesenescence markers on peripheral T cells were examined. Lastly, the repertoire and diversity of the antigen receptor of T cells were determined. Using flow cytometry, T cell phenotypes involved in the regulation and activation of the immune response were found to be significantly increased in children with symptomatic malaria compared to asymptomatic and healthy controls. In addition, levels of two regulatory and activation phenotypes could explain 68% of the variation in parasitaemia found in the asymptomatic and symptomatic groups. Also, the expression of T cell senescent and exhaustive markers were significantly upregulated in the symptomatic malaria children compared to the asymptomatic and healthy children. Importantly, levels of CD4PD-1 positively correlated with parasitaemia and could predict inflammation. Furthermore, applying high throughput sequencing on the T cell receptor β chain from healthy, asymptomatic, uncomplicated and severe malaria-infected children, the selective usage of specific Vβ and Jβ gene segments were observed in each group. Interestingly, the CDR3 nucleotide length in the severe malaria group was found to be significantly shortened, a characteristic shared with other autoimmune diseases. Also, increased repertoire diversity as well as significant sharing of TCR sequences was common in the asymptomatic cohort which seemed to be influenced by recombination events. Applying the GLIPH (grouping of lymphocyte interactions by paratope hotspots) algorithm, T cell receptors from the diseased groups could be clustered based on their antigenic specificity. Additionally, the binding motifs of TCR β chains were predicted. The data presented here, shows that malaria infection drives T cell expansion. However, T cell clones that are publicly shared as observed in the asymptomatic cohort may have an added advantage of being cross reactive to various P. falciparum antigens. These antigen-specific clones maybe able to maintain low parasitaemia levels providing favourable outcome by alleviating disease symptoms. In summary, studying T cells through quantitative approaches, accelerate the determination of T cell protective signatures and inform the choice of developing interventions.