Prasida Holla

Recently approved malaria pre-erythrocytic vaccines are important public health tools for the reduction of malaria morbidity and mortality. However, these vaccines have shown reduced protection in some of the most vulnerable age groups, require multiple dosing, and have the potential for parasite escape. These limitations have motivated the development of vaccines targeting multiple stages of the parasite life cycle. To ensure a robust pipeline, pre-clinical evaluation of additional malaria vaccine candidate antigens remains a key priority. The P. falciparum cysteine protease inhibitor falstatin can limit the cysteine protease activity of falcipains, controlling proteolysis at the site of merozoite entry on erythrocytes, thereby facilitating parasite invasion. Antibodies against falstatin have been shown to reduce the ability of merozoites to invade erythrocytes, raising the possibility that naturally acquired antibody responses could confer a similar advantage to the host. Here, IgG responses to full-length falstatin (falstatin _FL ) and two smaller, conserved regions (falstatin _35–68 and falstatin _289–335 ) were determined at the pre-malaria season baseline in a prospective cohort study conducted in Kalifabougou, Mali. The presence of anti-falstatin IgG was not associated with reduced risk of clinical malaria overall. However, baseline falstatin seropositivity predicted a non-statistically significant reduction in risk of febrile malaria within the first 30 days after incident parasitemia (log-rank statistic 3.2, P = 0.072). Using in vitro growth inhibitory assays, neither affinity-purified anti-falstatin _289–335 IgG nor affinity-purified anti-falstatin _FL IgG consistently inhibited P. falciparum blood-stage parasites. Larger cohort studies would be needed to confirm whether pre-existing anti-falstatin antibodies can delay the progression of malaria symptoms after blood-stage infection.

Chronic infectious diseases have a substantial impact on the human B cell compartment including a notable expansion of B cells here termed atypical B cells (ABCs). Using unbiased single-cell RNA sequencing (scRNA-seq), we uncovered and characterized heterogeneities in naïve B cell, classical memory B cells, and ABC subsets. We showed remarkably similar transcriptional profiles for ABC clusters in malaria, HIV, and autoimmune diseases and demonstrated that interferon-γ drove the expansion of ABCs in malaria. These observations suggest that ABCs represent a separate B cell lineage with a common inducer that further diversifies and acquires disease-specific characteristics and functions. In malaria, we identified ABC subsets based on isotype expression that differed in expansion in African children and in B cell receptor repertoire characteristics. Of particular interest, IgD⁺IgM^lo and IgD⁻IgG⁺ ABCs acquired a high antigen affinity threshold for activation, suggesting that ABCs may limit autoimmune responses to low-affinity self-antigens in chronic malaria.

T cells exposed to persistent antigen in the inflammatory environment of chronic infections often show progressive loss of effector functions, high expression of inhibitory receptors and distinct transcriptional programs. T cells in this functional state are termed “exhausted” and T cell exhaustion is associated with inefficient control of infections. A remarkably similar scenario has been described for B cells during chronic infections in humans, including malaria, in which case a subpopulation of atypical memory B cells (MBCs) greatly expands and these MBCs show attenuation of B cell receptor signaling, loss of the B cell effector functions of antibody and cytokine production, high expression of inhibitory receptors and distinct transcriptional profiles. The expansion of these MBCs is also associated with inefficient control of infections. Despite the similarities with exhausted T cells we speculate that at least in malaria, atypical MBCs may not be exhausted but rather may be functional, possibly even beneficial. Our recent results suggest that we simply may not have known how to ask an atypical MBC to function. Thus, exhaustion may not be in the human B cell's vocabulary, at least not in malaria.

Protective antibody responses to vaccination or infection depend on affinity maturation, a process by which high-affinity germinal center (GC) B cells are selected on the basis of their ability to bind, gather, and present antigen to T follicular helper (T_fh) cells. Here, we show that human GC B cells have intrinsically higher-affinity thresholds for both B cell antigen receptor (BCR) signaling and antigen gathering as compared with naïve B cells and that these functions are mediated by distinct cellular structures and pathways that ultimately lead to antigen affinity– and T_fh cell–dependent differentiation to plasma cells. GC B cells bound antigen through highly dynamic, actin- and ezrin-rich pod-like structures that concentrated BCRs. The behavior of these structures was dictated by the intrinsic antigen affinity thresholds of GC B cells. Low-affinity antigens triggered continuous engagement and disengagement of membrane-associated antigens, whereas high-affinity antigens induced stable synapse formation. The pod-like structures also mediated affinity-dependent antigen internalization by unconventional pathways distinct from those of naïve B cells. Thus, intrinsic properties of human GC B cells set thresholds for affinity selection.

Many chronic infections, including malaria and HIV, are associated with a large expansion of CD21⁻CD27⁻ ‘atypical’ memory B cells (MBCs) that exhibit reduced B cell receptor (BCR) signaling and effector functions. Little is known about the conditions or transcriptional regulators driving atypical MBC differentiation. Here we show that atypical MBCs in malaria-exposed individuals highly express the transcription factor T-bet, and that T-bet expression correlates inversely with BCR signaling and skews toward IgG3 class switching. Moreover, a longitudinal analysis of a subset of children suggested a correlation between the incidence of febrile malaria and the expansion of T-bet^hi B cells. The Th1-cytokine containing supernatants of malaria-stimulated PBMCs plus BCR cross linking induced T-bet expression in naïve B cells that was abrogated by neutralizing IFN-γ or blocking the IFN-γ receptor on B cells. Accordingly, recombinant IFN-γ plus BCR cross-linking drove T-bet expression in peripheral and tonsillar B cells. Consistent with this, Th1-polarized Tfh (Tfh-1) cells more efficiently induced T-bet expression in naïve B cells. These data provide new insight into the mechanisms underlying atypical MBC differentiation. Antibodies are proteins in blood that help kill microbes such as viruses, bacteria and parasites. Antibodies are produced by B cells with the help of T follicular helper (Tfh) cells. Some microbes for which we have no effective vaccines, such as HIV and malaria, establish chronic infections that are not cleared by the immune system. These chronic infections are associated with ‘atypical’ B cells that are less able to produce antibodies. We studied blood samples of malaria-exposed children to understand why normal B cells become atypical B cells. We found that atypical B cells express high levels of T-bet—a protein that is important for determining the fate of other types of immune cells. Children who frequently got malaria had more T-bet expressing B cells than children who rarely got malaria. We also found that malaria parasites cause immune cells to secrete inflammatory substances that cause normal B cells to express T-bet. Similarly, the inflammation-prone Tfh cells that malaria activates, which are relatively poor B cell helpers, also caused normal B cells to express T-bet. This study helps us understand why atypical B cells arise during chronic infections—information that could lead to strategies to improve antibody responses through vaccination.