Humoral immunity is important in limiting clinical disease in malaria, yet the longitudinal B cell response to infection remains unclear. We performed a 1-year prospective study in patients treated for acute Plasmodium falciparum malaria for the first time or with previous exposure to the disease. Using an unbiased exploratory approach with mass cytometry, followed by targeted flow cytometry, we found that approximately 80% of mature B cells that proliferated in response to acute infection expressed CD11c. Only approximately 40% of CD11c ⁺ B cells displayed an atypical B cell phenotype, with the remaining cells primarily made up of activated and resting memory B cells. The CD11c ⁺ B cells expanded rapidly following infection, with previous exposure to malaria resulting in a significantly larger increase compared with individuals with primary infection. This was attributed to an expansion of switched CD11c ⁺ B cells that was absent in primary infected individuals. The rate of contraction of the CD11c ⁺ B cell compartment was independent of previous exposure to malaria and displayed a slow decay, with a half-life of approximately 300 days. Collectively, these results identify CD11c as a marker of B cells responding to malaria and further highlight differences in primary and secondary B cell responses during infection.

In northern Europe, tick-borne diseases such as Lyme borreliosis (LB) and tick-borne encephalitis (TBE) are well known. The actual incidence of Babesia infections, however, has remained elusive. In this study, the prevalence of antibodies against two Babesia spp. was investigated in a cohort of patients that were seropositive for Borrelia (B.) burgdorferi sensu lato (s.l.). Data were compared to a control group of healthy individuals. Sera were collected from 283 individuals residing in the southernmost region of Sweden, Skåne County. Almost one third of the sera were from patients with a confirmed seropositive reaction against B. burgdorferi s.l. All sera samples were assessed for IgG antibodies against Babesia (Ba.) microti and Ba. divergens by indirect fluorescent antibody (IFA) assays. Seropositive IgG titers for at least one of the Babesia spp. was significantly more common (p < 0.05) in individuals seropositive for Borrelia (16.3%) compared to the healthy control group (2.5%). Our findings suggest that Babesia infections may indeed be quite common among individuals who have been exposed to tick bites. Furthermore, the results indicate that human babesiosis should be considered in patients that show relevant symptoms; particularly for splenectomized and other immunocompromised individuals. Finally, the data challenges current blood transfusion procedures and highlights the current lack of awareness of the parasite in northern Europe.

Understanding the functional characteristics of naturally acquired antibodies against P. falciparum merozoite antigens is crucial for determining the protective functions of antibodies. Affinity (measured as k_d) of naturally acquired antibodies against two key targets of acquired immunity, EBA175 and PfRh2, was determined using Surface Plasmon Resonance (SPR) in a longitudinal survey in Nigeria. A majority of the participants, 79% and 67%, maintained stable antibody affinities to EBA175 and PfRh2, respectively, over time. In about 10% of the individuals, there was a reciprocal interaction with a reduction over time in antibody affinity for PfRh2 and an increase for EBA175. In general, PfRh2 elicited antibodies with higher affinity compared to EBA175. Individuals with higher exposure to malaria produced antibodies with higher affinity to both antigens. Younger individuals (5–15 years) produced comparable or higher affinity antibodies than adults (>15 years) against EBA175, but not for PfRh2. Correlation between total IgG (ELISA) and affinity varied between individuals, but PfRh2 elicited antibodies with a higher correlation in a majority of the participants. There was also a correlation between antibody inhibition of erythrocyte invasion by merozoites and PfRh2 affinity. This work gives new insights into the generation and maintenance of antibody affinity over time.

B-cells are essential in immunity against malaria, but which sub-sets of B-cells specifically recognize Plasmodium falciparum and when they appear is still largely unknown. Using the flow cytometry technique for detection of P. falciparum specific (Pf+) B-cells, this study for the first time measured the development of Pf+ B cell (CD19+) phenotypes in Ugandan babies from birth up to nine months, and in their mothers. The babies showed increases in Pf+ IgG memory B-cells (MBCs), atypical MBCs, and plasma cells/blasts over time, but the proportion of these cells were still lower than in the mothers who displayed stable levels (5, 18, and 3%, respectively). Pf+ non-IgG+ MBCs and naïve B-cells binding to P. falciparum antigens were higher in the babies compared to the mothers (12 and 50%). In ELISA there was an increase in IgG and IgM antibodies over time in babies, and stable levels in mothers. At baby delivery, multigravidae mothers had a higher proportion of Pf+ IgG MBCs and less Pf+ naïve B-cells than primigravidae mothers. In newborns, naïve B-cells are a major player in recognizing P. falciparum. In adults, the high proportion of Pf+ atypical MBCs suggests a major role for these cells. Both in infants and adults, non-IgG+ MBCs were higher than IgG MBCs, indicating that these cells deserve more focus in future.

Plasmodium falciparum EBA175 and PfRh2 belong to two main families involved in parasite invasion, and both are potential vaccine candidates. Current knowledge is limited regarding which target antigens and subclasses of antibodies are actually important for protection, and how naturally acquired immunity is achieved. Repeated blood samples were collected from individuals in Nigeria over a period of almost one year. ELISA was used to analyse subclasses of IgG responses. For both EBA175 (region III-V) and (a fragment of) PfRh2, the dominant antibody responses consisted of IgG1 and IgG3 followed by IgG2, while for PfRh2 there was also a relatively prominent response for IgG4. High levels of IgG1, IgG2 and IgG3 for EBA175 and total IgG for PfRh2 correlated significantly with a lower parasitaemia during the study period. Children with HbAS had higher levels of some subclasses compared to children with HbAA, while in adults the pattern was the opposite. The half-lives of IgG2 and IgG4 against EBA175 were clearly shorter than those for IgG1 and IgG3. EBA175 and PfRh2 are potential targets for protective antibodies since both correlated with lower parasitaemia. The shorter half-lives for IgG2 and IgG4 might explain why these subclasses are often considered less important in protection against malaria. Triggering the right subclass responses could be of critical importance in a successful vaccine. Further studies are needed to evaluate the role of haemoglobin polymorphisms and their malaria protective effects in this process.