Human cytomegalovirus pp65-and immediate early 1 antigen-specific HLA class I-restricted cytotoxic T cell responses induced by cross-presentation of viral antigens

Dendritic cells (DCs) play a pivotal role in the development of anti-viral CD8(+) CTL responses. This is straightforward if they are directly infected with virus, but is less clear in response to viruses that cannot productiv ely infect DCs. Human CMV (HCMV) shows strain-specific cell tropism: fibrob last (Fb)-adapted laboratory strains (AD169) and recent clinical isolates d o not infect DCs, whereas endothelial cell-adapted strains (TB40/E) result in productive lytic DC infection. However, we show here that uninfected DCs induce CD8(+) T cell cytotoxicity and IFN-gamma production against HCMV pp 65 and immediate early 1 Ags following in vitro coculture with HCMV-AD169-i nfected Fbs, regardless of the HLA type of these Fbs. CD8(+) T cell stimula tion was inhibited by pretreatment of DCs with cytochalasin B or brefeldin A, indicating a phagosome/endosome to cytosol pathway. HCMV-infected Fbs we re not apoptotic as measured by annexin V binding, and induction of apoptos is of infected Fbs in vitro did not augment CTL induction by DCs, suggestin g a mechanism other than apoptosis in the initiation of cross-presentation. Furthermore, HCMV-infected Fbs provided a maturation signal for immature D Cs during coculture, as evidenced by increased CD83 and HLA class II expres sion. Cross-presentation of HCMV Ags by host DCs enables these professional APCs to bypass some of the evasion mechanisms HCMV has developed to avoid T cell recognition. It may also serve to explain the presence of immediate early 1 Ag-specific CTLs in the face of pp65-induced inhibition of Ag prese ntation at the level of the infected cell.