Characterisation of the interaction between the Hepatitis B virus core antigen and B cells

Abstract

The Hepatitis B virus (HBV) is a major cause of severe human liver disease globally. There are around 350 million persistent HBV carriers with a risk of development of liver disease including cirrhosis and hepatocellular carcinoma. Hepatitis B virus is a non-cytopathic virus and the common belief is that liver disease is host immune system mediated. HBV belongs to the Hepadnaviridae virus family. The intact virion consists of the viral DNA genome associated with a viral DNA polymerase that is encapsidated and surrounded by an outer lipid envelope. The mature nucleocapsid exhibits hepatitis B core antigenicity (HBcAg). The secreted counterpart of HBcAg, termed the hepatitis B e antigen (HBeAg), shares a 150 amino acid homology with HBcAg. The HBcAg seems to be the most immunogenic protein of the proteins encoded by HBV. The reason for this is not fully understood.

It has been shown that HBcAg preferentially primes Th1-like immune responses, whereas HBeAg primes more Th2-skewed responses. To further study this issue we developed a flow cytometry based technique for multiparameter analyses of HBcAg-specific cytokine responses and cell Proliferation in mice. We found that there is no definite relation between the levels of cytokines produced and the number of cytokine producing T cells. Thus, several types of assays should be used to characterize the Th1- and Th2-phenotype of HBV specific immune responses. It is clear that HBcAg interacts with B cell but the basis for the interaction is not known. High levels of anti-HBc IgM antibodies can be detected in acute stages of the disease and during periods of acute exacerbation in the chronic hepatitis.

It has been proposed that B cells may function as primary antigen presenting cells for HBcAg. We were therefore interested to further characterize the interaction between HBcAg and B cells. The precursor frequency of naive human and murine B cells binding to HBcAg was determined. The interaction between HBcAg and human B cells was studied in a mouse model repopulated with human peripheral blood leukocytes. We found that around 5-15% of the naïve human and murine B cell population bind HBcAg. This unusually high frequency of HBcAg-binding B cells led us to assumption that HBcAg would interact with B cell receptors (BCRs) encoded by a restricted set of V germline gene families. Indeed, we found that HBcAg bound to a linear motif corresponding to the FR1/CDR1 junction region on immunoglobulin variable domains. Using mutant HBcAg particles the region on HBcAg involved in the interaction with the BCR could be mapped to the tip of the spike region of HBcAg.

We next analyzed the effects on the naïve B cell induced by binding of HBcAg. After binding of HBcAg the B cell up-regulates the activation marker CD69, the co-stimulatory molecule B7- 2, and start producing IFN-gamma. Thus, HBcAg can induce a marginal activation in large population of naïve B cells that become efficient antigen presenting cells for T helper cells. However, little is know about the regulatory role of B cells on cytotoxic T lymphocytes. To elucidate this question wild type and B cell deficient mice were immunized with endogenous (DNA immunization) and exogenous (recombinant protein) HBcAg. Endogenously produced HBcAg primed HBcAg-specific CTLs independent of B cells. In contrast, exogenous HBcAg could only prime CTLs in the presence of B cells, suggesting a regulatory role for B cells in the priming of HBcAg-specific CTLs.

Collectively, HBcAg has the unique ability to bind and activate naïve murine and human B cells existing at a high precursor frequencies. The presentation of HBcAg through B cells may shape CD4+ T cell response of the host and may regulate priming of CD8+ T cells. The reason why HBV has targeted HBcAg to B cells remains to be determined.