Mobilizing monocytes to cross-present circulating viral antigen in chronic infection

Abstract

Selection of antigens for therapeutic vaccination against chronic viral infections is complicated by pathogen genetic variations. We tested whether antigens present during persistent viral infections could provide a personalized antigenic reservoir for therapeutic T cell expansion in humans. We focused our study on the HBV surface antigen (HBsAg), which is present in microgram quantities in the serum of chronic HBV patients. We demonstrated by quantitative fluorescent microscopy that, out of 6 professional APC populations in the circulation, only CD14 monocytes (MNs) retained an HBsAg depot. Using TCR-redirected CD8 + T cells specific for MHC-I-restricted HBV epitopes, we showed that, despite being constantly exposed to antigen, ex vivoisolated APCs did not constitutively activate HBV-specific CD8 + T cells. However, differentiation of HBsAg + CD14 MNs from chronic patients to MN-derived DCs (moDCs) induced cross-presentation of the intracellular reservoir of viral antigen. We exploited this mechanism to cross-present circulating viral antigen and showed that moDCs from chronically infected patients stimulated expansion of autologous HBV-specific T cells. Thus, these data demonstrate that circulating viral antigen produced during chronic infection can serve as a personalized antigenic reservoir to activate virus-specific T cells. IntroductionTherapeutic vaccination for chronic infections, be it recombinant antigens, peptides, viral vectors, DNA, or DCs, are hindered by the need to select appropriate antigens. It is a major complicating factor due to the evolutionary diversity that pathogens have developed in response to selective forces exerted by individual (immune response) or environmental (drugs, vectors) factors. Moreover, peptides covering conserved regions for vaccination are HLA restricted and can only be applied to selected patients with the appropriate HLA. As a result, recombinant antigens or DNA vectors coding pathogen proteins may misdirect the intended immune response due to differences between the infectious pathogen and the antigen sequence utilized for vaccination.A hallmark of many chronic infections is the constant production of pathogen proteins. This is particularly evident in HBV infection, where viral titers can reach 10 9 -10 10 virions/ml in the serum. The HBV surface antigen (HBsAg) is produced in excess of whole virions and reaches concentrations well into the μg/ml range (1). While persistently present viral antigen is generally considered a negative factor (2), the abundance of endogenously produced viral antigen could be internalized by different cell types. Proper activation of cells internalizing antigen in the circulation of chronic patients could provide a target for therapeutic vaccination and stimulate T cells with antigen customized to the patient's viral genome.HBV does not infect or productively replicate in human PBMCs (3), and systematic analysis of cells capable of internalizing circu-

Dissecting the dendritic cell controversy in chronic hepatitis B virus infection

“…29,30 We demonstrated that HBsAg 1 CD14 monocytes, when differentiated to moDCs, cross-presented in vivo-captured antigen and expanded autologous HBV-specific CD4 and CD8 T cells. 17 Similarly, other studies have shown that autologous moDCs loaded with recombinant HBV antigens were capable of stimulating autologous T-cell proliferation. [37][38][39] Because of their stimulatory capacity, moDCs loaded with recombinant antigens or synthetic peptides have been administered to patients in two clinical trials.…”

“…A similar observation was made in terms of mDC cytokine profiles, in which the IL-12p70 and IL-10 production was equal between HBV patients and healthy donors. 12,[14][15][16][17] Exceptions are present, however. Zhang et al 12 reported a lower frequency of mDCs in the blood of immune active pediatric patients, but not in those who were immune-tolerant.…”

“…CD14 1 MN (monocyte) circulating in patients internalize and retain a depot of HBV antigens. 17 This seems to have little impact on their ability to respond to inflammatory stimuli 27 or induce T-cell proliferation, 17 suggesting that the monocyte, i.e., the moDC precursor, function is intact. There have only been a few studies in which monocytes were isolated from chronic HBV patients, differentiated to moDCs and compared with healthy moDCs (Table 3).…”

“…43 We have shown that DCs and monocytes in the peripheral blood do not constitutively present circulating antigen to virus-specific CD8 T cells, 17 but little is known about presentation to CD4 T cells or antigen presentation by liver or splenic DCs in chronic HBV patients. Studying antigen processing in primary mDCs from chronic HBV patients will be important to assess their potential for activating virus-specific T cells.…”

Host–virus interactions in hepatitis B and hepatitis C infection

“…T cells in patients with chronic HBV infection [45]. In another study, monocyte-derived DCs retaining HBsAg recovered from chronically infected patients were able to increase the number of autologous HBV-specific T cells [46], suggesting that they are a promising adjuvant in vivo. A DNA vaccine could be another candidate: a fusion DNA encoding mouse DEC-205 linked with HBsAg successfully induced T cell and antibody responses against HBsAg in HBV transgenic mice [47].…”