Antigen persistence and the control of local T cell memory by migrant respiratory dendritic cells after acute virus infection

Acute viral infections induce robust adaptive immune responses resulting in virus clearance. Recent evidence suggests that there may be depots of viral antigen that persist in draining lymph nodes (DLNs) after virus clearance and could, therefore, affect the adaptive immune response and memory T cell formation. The nature of these residual antigen depots, the mechanism of antigen persistence, and the impact of the persistent antigen on memory T cells remain ill defined. Using a mouse model of influenza virus infection of the respiratory tract, we identified respiratory dendritic cells (RDCs) as essential for both sampling and presenting residual viral antigen. RDCs in the previously infected lung capture residual viral antigen deposited in an irradiation-resistant cell type. RDCs then transport the viral antigen to the LNs draining the site of infection, where they present the antigen to T cells. Lastly, we document preferential localization of memory T cells to the DLNs after virus clearance as a consequence of presentation of residual viral antigen by the migrant RDC.The induction of the adaptive immune response to an infectious agent consists of a defined series of events prompted by invasion of the body by the organism, followed by replication of the organism and the subsequent initiation of an innate and adaptive immune response. For organisms that enter at a body surface and confine replication primarily to that site, for example, infection of the respiratory tract by seasonal type A influenza, innate responses are triggered by infection of cells at the body surfaces and activation of sentinel innate immune cells in the surrounding tissues. Induction of the adaptive immune response typically requires the uptake/infection by the organism or uptake of its constituents by tissue-resident professional APCs, most notably immature DCs (Banchereau and Steinman, 1998; Lambrecht et al., 2001; de Heer et al., 2005). These professional APCs transport the organism or its products to secondary lymphoid organs, usually LNs draining the site of infection (draining LN [DLN]) where adaptive immune (T and B cells) responses are initiated. In the case of experimental mouse influenza infection, at least two distinct subsets of respiratory DCs (RDCs) have been implicated as critical APCs in the induction of primary T cell responses to this virus (GeurtsvanKessel et al., 2008; Kim and Braciale, 2009).The outcome of the interaction between an invading microorganism and the immune system is either clearance of the agent or the development of chronic persistent infection. Viruses such as HIV, HCV, and, in the mouse model, LCMV, can produce chronic persistent infection with high titers of the virus because of the ability of these viruses to subvert or suppress the host immune response (Ahmed et al., 1984; Bevan and Braciale, 1995; Letvin and Walker, 2003; Wherry et al., 2003; Rehermann and Nascimbeni, 2005). Persistence of viral antigen at high levels in these infections may contribute to the dysregulation of the adaptive immune response observed in these infections (Shin and Wherry, 2007). In contrast, acute infection with many viruses results in rapid virus clearance (even with an initial high level of virus replication) and, presumably, elimination of virus-infected cells by the action of the adaptive and innate immune response.Recently, however, evidence has emerged to suggest that after acute viral infection viral antigen can be detectable for an extended period, i.e., weeks to months, after clearance of infectious virus. This phenomenon is not only observed with viruses capable of establishing persistent infection in vitro, for example, respiratory syncytial virus (RSV; P’ringle et al., 1978; Schwarze et al., 2004) and Sendai virus (Mori et al., 1995), but has also been observed for classically lytic viruses, such as type A influenza (Jelley-Gibbs et al., 2005; Zammit et al., 2006) and VSV (Turner et al., 2007), which are not believed to produce chronic persistent infection of cells. In these instances, viral antigen was detected using the proliferative response of adoptively transferred TCR transgenic (Tg) T cells into previously infected animals as a sensitive measure of antigen persistence. Although the nature and the mechanism of antigen persistence in these instances was not defined, several studies provided evidence that the presentation of this residual viral antigen to T cells may influence the quality of the memory T cell response to infection (Jelley-Gibbs et al., 2005, 2007; Zammit et al., 2006; Turner et al., 2007; Woodland and Kohlmeier, 2009).In several studies reporting persistence of viral antigen after acute infection and subsequent virus clearance, the reservoir of antigen (as detected by adoptive transfer of virus-specific T cells) was localized within secondary lymphoid organs, most notably the LN draining the site of infection (Jelley-Gibbs et al., 2005, 2007; Zammit et al., 2006). In this paper, we describe a series of experiments to investigate the underlying mechanism of the persistence of antigen presentation after respiratory tract infection with type A influenza virus. We found that a reservoir of viral antigen (i.e., RNA and protein) is present for an extended period after an acute influenza virus infection at the site of infection (i.e., the lungs) and localized to both nonhematopoietic (CD45⁻) and hematopoietic (CD45⁺) cell types within residual mild lung inflammatory foci. We further demonstrate that the residual viral antigen deposited in the previously infected lung is captured and transported by RDC to the regional LN, where the migrant RDC function in presenting the residual antigen to T cells. In addition, we provide evidence that lung follicular DCs (FDCs) may play an important role in regulating the response of RDC. Finally, our results suggest that as a consequence of presentation of residual viral antigen by the migrant RDC to T cells in the LNs draining the site of infection, there is preferential localization of memory T cells to the DLN after virus clearance. The implications of these observations are discussed.