A vaccine to combat malaria is a highly desirable public health tool to reduce morbidity and mortality in African children. In order to achieve this goal it will be important to gain a detailed understanding of the impact of malaria on the generation and maintenance of immunological memory, the basis of all vaccines. This project represents a collaborative effort between Dr. Pierce, Dr. Louis Miller, Dr. Peter Crompton and with scientists at the Malaria Research and Training Center (MRTC) at the University of Mali. Our goal is to gain an understanding of the generation, maintenance and activation of immunological memory in response to natural malaria infection. Our analyses to date have focused on the acquisition of antibody (Ab) memory in malaria because Abs were shown in passive transfer experiments to be sufficient to control both fever and parasitemia in malaria. In humans, B cell memory is encoded both in long-lived memory B cells and in plasma cells that reside in the bone marrow. The mechanisms by which memory B cells or plasma cells are generated and maintained over a life time are not known. Current evidence, primarily from serological epidemiological studies, indicates that immunological memory to malaria is slow to be acquired, incomplete and short lived. Thus, despite chronic exposure to Plasmodium falciparum (Pf) from birth from infectious mosquito bites, children in endemic areas do not acquire immunity that protects them from severe disease until the age of five. Consequently, children under five years of age are susceptible to severe disease that accounts for nearly one million deaths each year in Africa alone. Acquisition of immunity that protects against severe disease but not against mild disease is acquired prior to adolescence and an immunity sufficient to prevent disease but not to eliminate parasites is acquired only in adolescence to early adulthood. We determined the acquisition of Pf-specific Abs in a cohort of children and young adults in Kambila using a proteome chip containing approximately 25% of the Pf proteome. Our results indicate that the intensity and complexity of the malaria-specific antibody response increased with age and that the intensity and complexity of the serum antibodies at the beginning of the malaria season predicts protection from malaria. We also observed that the complexity and intensity of antibodies transiently rose during the season but most of the increases were lost by six months and did not predict protection. Thus, the long-lived Pf-specific Abs increased slowly year by year in a step-wise fashion. We also identified an antibody signature associated with malaria immunity. Antibodies specific for a subset of 49 Pf proteins were found to be significantly increased in immune children as compared to nonimmune children. The increased antibody responses specific for the top five of these 49 proteins proved to be a highly sensitive and specific indicator of malaria immunity. Our analysis of the acquisition of malaria-specific MBCs in the same cohort of children and young adults in Kambila showed that Pf-specific memory B cells increased with age but only in a gradual stepwise fashion over years of exposure to malaria. We observed that the number of malaria-specific MBCs transiently increased after the first malaria infection. The increase in MBCs was accompanied by a short term rise in the levels of malaria-specific antibody. Although classical antigen-specific MBCs were generated in response to malaria infections we also observed a large increase in the number of atypical FcRL4+ MBCs. FcRL4 was first described to mark a unique population of MBCs in healthy individuals in mucosal lymphoid tissues in close proximity to epithelial boundaries in the body, site of pathogen invasion. These FcRL4+ MBCs had distinctive functional characteristics and gene expression profile. Because of their location FcRL4+ tissue MBCs are thought to play a role in antibody responses to pathogens. Subsequently FcRL4+ MBCs were identified in large numbers in the peripheral blood of HIV-infected viremic individuals suggesting that FcRL4+ MBCs may differentiate in response to chronic antigenic stimulation. We found that FcRL4+ MBCs could represent up to 30% of B cells in Malian children as young as two years of age. We initiated studies to determine the Ab repertoire of conventional FcRL4- and FcRL4+ MBCs. Understanding the origin and function of these atypical MBCs should be of significant interest in the development of malaria vaccines. Our current hypothesis to explain the slow acquisition of Ab memory and the rapid acquisition of FcRL4+ MBCs is that Pf infections disrupt the normal mechanisms by which memory is generated, maintained or activated. We presume that such disruption is mediated through interactions between Pf and human host immune system proteins. However, to date no such interactions have been identified. To fill this gap is knowledge we carried out an extensive yeast two hybrid screen between 150 Pf proteins anticipate to be exposed to the human immune system and 12,000 human open reading frames. We identified several potential interactions and have made significant progress on one. We showed that the 33kD fragment of Pf merozoite surface protein 1 (MSP133), an abundant merozoite surface protein that is shed during red blood cell invasion, binds to S100P, a member of S100 family, several of which are pro-inflammatory, damage associated molecular pattern proteins. MSP133 binds selectively to S100P with high-affinity and blocks S100P-induced NFkappaB activation in monocytes and chemotaxis and superoxide production in neutrophiles. Remarkably, S100P binds to both dimorphic alleles of MSP1, estimated to have diverged over 27 million years ago, suggesting an ancient conserved relationship between these parasite and host proteins. We postulate that MSP133 is released by Pf merozoites to attenuate potentially damaging inflammatory responses for the benefit of both the parasite and the host. Over the last year we also explored the relationship between Pf infections and autoimmune disease. Plasmodium falciparum has exerted tremendous selective pressure on genes that improve survival in severe malarial infections. Systemic lupus erythematosus (SLE) is an autoimmune disease that is six to eight times more prevalent in women of African descent as compared to women of European descent. We provided evidence that a genetic susceptibility to SLE protects against cerebral malaria. Mice that are prone to SLE due to a deficiency in FcgammaRIIB, or to overexpression of TLR7, are protected from death due to cerebral malaria. Protection appears to be by immune mechanisms that allow SLE-prone mice to better control their overall inflammatory responses to parasite infections. These findings suggest that the high prevalence of SLE in women of African descent, living outside of Africa, may be due to the inheritance of genes that are beneficial in the immune control of cerebral malaria, but in the absence of malaria, contribute to autoimmune disease.
Showing the most recent 10 out of 35 publications
