Impact of malaria on B-cell homeostasis and Epstein-Barr virus reactivation : endemic Burkitt's lymphoma pathogenesis

Abstract

Over recent years, the concept that many diseases can be aetiologically linked to infection by more than one pathogen has gained increased attention and awareness. Plasmodium falciparum (P.f) and Epstein-Barr virus (EBV) are recognized co-factors in the genesis of endemic Burkitt s lymphoma (eBL), a high grade B cell malignancy, accounting for up to 74% of childhood cancers in equatorial Africa. In this thesis, we have investigated the interactions existing between P.f. and EBV that could lead to the emergence of eBL. A special emphasis was given to the effect of malarial antigens on B cell homeostasis and EBV reactivation.

During intra-erythrocytic proliferation of P.f, parasite-derived proteins are successively expressed, exported and presented at the surface of the human red blood cell membrane. The cysteine-rich interdomain region 1 alpha (CIDR1alpha) of the P.f erythrocyte membrane protein 1 (PfEMP1) harbors a multi-adhesive phenotype able to bind to CD36, CD31 and immunoglobulins (Ig). This led us to investigate the effect of P.f infected erythrocyte (IE) and CIDR1alpha, on purified human B cell preparations. Both IE and CIDR1alpha bound to non-immune B cells and induced polyclonal activation accompanied by the production of cytokines and antibodies. This effect was partially mediated by B cell surface immunoglobulins (a constitutive part of the B cell receptor). Nevertheless, the different gene expression profiles obtained comparing the activation signature of CIDR1alpha and anti-Ig stimuli suggested that other molecules/signaling pathways were implicated in this activation process. CIDR1alpha preferentially activated the memory B cell compartment and was capable of rescuing germinal center B cells from spontaneous apoptosis, promoting cell cycle entry.

Polyclonal B cell activation is a prominent feature of malaria. We have identified the CIDR1alpha domain of PfEMP1 as a T cell-independent antigen that induces polyclonal B cell activation, shedding light on a possible molecular mechanism leading to hyper-gammaglobulinemia during malaria infection.

After primary infection, EBV establishes a life long persistence in the host, residing in a latent state in memory B cells. P.f infection is associated with elevated EBV loads. Different but not exclusive causative effects have been proposed: i) the impairment of the EBV-specific T cell response resulting from malaria-induced immunosuppression and ii) the expansion of the pool of EBV-carrying B cells resulting from malaria-induced polyclonal B cell activation. We and others have recently demonstrated that children living in malaria endemic areas have elevated EBV levels in the plasma indicating that the high EBV loads observed during malaria infection could also result from active viral replication. This led us to assess the impact of CIDR1alpha on EBV reactivation using the EBV positive BL cell line Akata as a model. Upon stimulation with CIDR1alpha, quantitative determination by real time PCR revealed an increased EBV load in the Akata cell cultures. The increased viral load resulted from lytic cycle reactivation as confirmed by experiments performed using an Akata line-based system in which the induction of lytic cycle was reflected by an enhanced expression of green fluorescent protein (GFP). CIDR1alpha stimulation led to an augmentation of GFP positive cells. Moreover, the virus production in CIDR1alpha-exposed cultures was directly proportional to the number of GFP-positive Akata cells (lytic EBV) and to increased expression of the EBV lytic promoter BZLF1. CIDR1alpha also induced the production of EBV in peripheral blood mononuclear cells derived from healthy donors and from children with eBL. Our results demonstrate that P.f antigens, such as CIDR1alpha, can directly drive an EBV latently infected B cell into lytic cycle.

We subsequently monitored the activity of all known herpes viruses (HHVs) infecting humans in saliva and plasma samples from children having acute malaria (day-0) and 14 days after they received anti-malaria treatment (day-14). Children with acute P.f malaria infection had elevated levels of circulating EBV, these levels being cleared after recovery. Acute malaria infection was not associated to an increased plasma load of HSV-1, CMV, HHV.6 or HHV-7, as compared to the control groups (malaria day-14 and malaria negative). However, we observed a profound reduction of HSV-1 levels in the saliva after anti-malarial treatment whereas the salivary loads of other HHVs, including EBV, were unchanged. Due to the low detection rates of HSV-2, VZV and HHV-8 in our study, we couldn t draw any significant conclusions on their activity during P.f infection. Taken together our results suggest the existence of an intimate link between malaria and EBV. The elevated EBV loads observed during malaria infection seems to result not only from an impairment of the EBV-specific T cell response and polyclonal B cell activation but also from viral reactivation directly driven by malarial antigens.

In conclusion, this thesis provides unique insights on the molecular mechanisms underlying polyclonal B cell activation and EBV reactivation during Plasmodium falciparum malaria infection and on how two pathogens can co-operate in lymphoma pathogenesis.