Adaptive immune responses mediated age-related Plasmodium yoelii 17XL and 17XNL infections in 4 and 8-week-old BALB/c mice

Malaria is a serious infectious disease, which cause of mortality and morbidity in tropical countries [1]. According to WHO report, malaria transmission was found in 91 countries,with Africa experiencing disproportionately high malaria cases (90% of the total) and accounting for 91% of total malaria deaths worldwide [2]. Notably, children under the age of 5 years are particularly vulnerable to plasmodium infection. More than two-thirds of malaria deaths (70%) occur in this age group [3]. Of the five Plasmodium species that infect humans, Plasmodium falciparum and Plasmodium vivax are the most common, and P. falciparum is the most virulent and responsible for the majority of deaths [2, 3]. In addition, the multiplicity of infection (MOI) varies depending on the overall prevalence of infection in the population, and the age of the individual [4, 5]. The young children are highly susceptible to clinical illness and high parasitemia, whereas the adults are highly resistant [4], resulting in a major difference in the spectrum of disease manifestations between children and adults [6]. Therefore, understanding the immunological mechanisms involved in susceptibility to different virulent Plasmodium species during infection in children or adulthood could contribute to the development of an immunologically based control strategy to prevent or treat this devastating disease.

Upon infection, anti-parasite immunity plays a pivotal role in removing the parasite from the blood. Firstly innate immunity is activated via complement system, innate lymphoid cells and dendritic cells (DCs), act to limit the acute phase of parasitemia, but are insufficient to clear the infection [7‐9]. When DCs present the processed antigen, adaptive immunity is activated. Direct cell cytotoxicity, cytokine secretion as well as anti-malarial antibody work together for effective parasite clearance [10‐13]. Childhoods and young children are more susceptible to malaria infection than adults worldwide [4]. Age-related changes in immune systems increased prevalence of asthma, nasal polyps and lung injury [14, 15]. However, whether differences in cellular and humoral immunity lead to this age-related infection profile remains unknown. Therefore, we used different virulent Plasmodium (lethal P.y17XL and non-lethal P.y17XNL) strains to infect4-week-old and 8-week-old BALB/c mice to mimic infancy and adulthood, respectively, in order to characterize the relationship between immune cell responses and age-related malaria infection among different age groups, and understand the mechanism of malaria immunity. We propose that the dynamics of MOI can be explained by a model of increasing acquired immunity to blood-stage infection with age.

Malaria infection is known to be age-related, with children being more susceptible than adults [17‐20]. This study aimed to investigate whether the susceptibility to malaria infection in children and adulthood is associated with cellular and humoral immune responses, using a mouse model of lethal P.y17XL and non-lethal P.y17XNL infections in different age groups. Children mice were found to be more susceptible to P.y17XNL infection, with higher parasitemia at various time points. The adult group was more resistant to P.y17XL infection with lower parasitemia during the early stage of malaria infection. Importantly, enhanced cellular and humoral immunity, especially MSP-1 specific antibody, might contribute to rapid clearance of malaria in the adult group.

Malaria infections have various clinical symptoms, including febrile, anemia, acidosis and end-organ failure. To be mentioned, the difference of clinical phenotypic correlated with parasite proliferation rates, which can be controled by erythrocyte and hemoglobin polymorphisms [21]. In addition, disease profile can be determined by the strain and host [22]. In this study, 4-week-old and 8-week-old mice were used to mimic infancy and adulthood, respectively. We successfully established the age-related malaria infection mouse model to study the age-related anti-malaria immunity. Compared with 8-week-old group, the survival rate and parasitemia at different time points indicated that the 4-week-old group was same to both lethal and non-lethal parasite infections. After non-lethal P.y17XNL infection, parasitemia was significantly higher in the 4-week-old mice than the 8-week-old mice during the acute and chronic stages of infection. After lethal P.y17XL infection, a significant difference in parasitemia was observed in the early stage of infection. In accordance with the parasitemia, enhanced Th1 immune responses were only observed in the early stage in adult mice after lethal P.y17XL infection and enhanced adaptive immune responses (Th1 and Th2) were detected in adult mice during non-lethal P.y17XNL infection. These data suggested that the difference in response to non-lethal and lethal Plasmodium infections was associated with the pattern of immune cell responses in the host. Thus, clinical phenotypes of malaria infections can be determined by age and immune states from host.

Similar to other infectious diseases, accumulating evidences have indicated that CD4⁺ T cells are essential to control malaria infection [23‐26]. Numerous studies have highlighted the role of Th1/Th2 cells or related signaling mechanisms in controlling malaria infection [27‐31]. In this study, enhanced Th1 and Th2 responses were displayed in 8-week-old mice after malaria infection. Significantly higher percentage of Th1/Th2 cells and level of IFN-γ/IL-4 were observed in the 8-week-old mice as compared to the 4-week-old mice. In vitro studies also showed an enhanced Th1 cell response, which indicated an important role of Th1/Th2 cell-mediated age-related anti-malarial response. However, many studies suggested a shift from Th2 to Th1 cell responses with age. Li et al. found that IFN-γ level increased with age but not Th-related transcription factors, while IL-4 expression in plasma and CD4⁺ splenocytes declined with age [32]. A shift from Th2 towards Th1 immune responses was also observed in children with tertian or tropical malaria infection [33]. These studies partly supported our conclusion that enhanced Th1 cells might contribute to malaria clearance during the early stage of plasmodium infection. However, we observed enhanced Th2 cells during the late stage/chronic stage of malaria infection. Further studies are needed to investigate if any shift exists during the early stage of malaria infection. In addition, follicular T helper (Tfh) cells are essential for Plasmodium infection clearance by activating germinal center B cell responses [34‐37]. Relative research found that the preferential localization of Tfh cells in the germinal center (GC) suggests a unique, intimate relationship between the Tfh cell and the B cell. Cytokines and cell-surface receptors provided by Tfh cells, as a kind of auxillary signal incompletely to keep GC B cells alive and proliferation via CD40L, IL-21 and IL-4 help [38, 39]. In this study, the percentage and absolute number of CD4⁺CXCR5⁺ Tfh cells peaked on day 5 p.i., and then decreased to normal level on day 10 p.i. in the 4-week-old mice. However, in the 8-week-old mice, the percentage and absolute number of CD4⁺CXCR5⁺ Tfh cells were significantly increased on day 10 p.i. as compared to the 4-week-old mice (Fig. 3e, f). Relative studies found that the addition of Tfh cells induces GC collapse, result for damage of B cells. Thus we speculated that GC perhaps has collapsed in young mice during the early stage of plasmodium infection, because Tfh cells increased rapidly. In addition, the GC is the primary site of B cell affinity maturation [39]. These studies supported our findings that the impaired function of antibody-secreting B cells and Tfh cells in childhood and children may account for their susceptibility to malaria infection.

We also observed a dampening of PD-1 signaling on activated CD4⁺ T cells after non-lethal P.y17XNL infection but not lethal P.y17XL infection in the 8-week-old mice. PD-1 co-inhibitory signaling was reported to regulate helper T cell differentiation and anti-Plasmodium humoral immunity [39], and PD-1 deficiency could enhance humoral immunity during malaria infection [40]. PD-1 was also a marker of T-cell exhaustion [41]. Several studies have also proven that chronic malaria infection drives T cell exhaustion through PD-1 signaling [42, 43]. Therefore, we speculated that during non-lethal infection, humoral immunity plays an essential role in the late stage of malaria clearance, perhaps correlated with enhanced PD-1 signaling on activated CD4⁺ T cells, which may help to drive CD4⁺ effector T cell exhaustion and promote persistent infection in children. Therefore, differences in PD-1 signaling could be observed in different age groups after non-lethal but not lethal malaria infection.

Several studies have confirmed that immune effector mechanisms are required to eliminate malarial parasites, and B cells secrete specific antibodies supported by Th2 cells, which can effectively remove the parasites to prevent the recidivation and recrudescence [44, 45]. Similarly, infusion of malaria hyperimmune serum resulted in rapid clearance of parasitized erythrocytes [45]. Merozoites proliferation from RBCs can be prevented by Anti-Plasmodium antibodies, depended on blocking cytoadherence to endotheliar capillary of iRBCs and promoting phagocytosis by mononuclear cells [46‐48]. However, researchers found that levels of antimalarial antibodies continue to increase significantly resulting from chronic exposure to infection [49], perhaps correlated with impaired establishment of B cell memory [50]. Thus in young children,we can found the short-lived antibody responses [51‐54]. In this study, we detected the levels of B cell-related total IgG, IgG1 and IgG2a in P.y17XNL-infected BALB/c mice. The results showed a difference in antibody production between adult and children mice, and the levels of total antibody might contribute to rapid clearance of malarial parasites in the adult group during the chronic stage of non-lethal P.y17XNL infection. Moreover, IgG1 and IgG3 antibodies against merozoite surface proteins (MSPs) are thought to be instrumental in protection, which is considered as a major vaccine candidate [55]. Therefore, we detected the levels of P.y MSP-1 specific antibody. Consistently, the dynamics of P.y MSP-1 specific antibody was the same as total antibody. These data implied that an enhanced antibody response during chronic stage of non-lethal P.y17XNL infection might contribute to rapid clearance of malaria in the adult group.

Taken together, the findings of this study revealed that in non-lethal P.y17XNL infection, higher burden of parasitemia in children mice were associated with weakened Th1 cellular immune responses, down-regulated humoral immunity with decreased percentage and number of Th2 and Tfh cells as well as lower level of antibody secretion and enhanced PD-1 signaling on activated CD4⁺ T cells. Higher resistance to lethal P.y17XL infection in the early stage in adult mice was associated with enhanced Th1 cellular immune responses and weakened PD-1 signaling on activated CD4⁺ T cells. These results provide a new insight on immune responses in malaria infection.

However, we have to consider a question: the expression of PD-1 on activated CD4⁺ T cells induced depletion of immune cells. On the one hand, depletion of immune cells induced down-regulation of anti-malaria immune response; on the other hand, exhaust of immune cells reduces the immunopathological injury in malaria-infected hosts. It is very difficult to regulate the dynamic balance between them. So there are many questions to deal with about using PD-1 antagonists to treat malaria infections.