Dendritic cells (DCs) are considered the most potent antigen-presenting cells (APCs), which directly prime or cross-prime MHC I-restricted cytotoxic T cells (CTLs). However, recent evidence suggests the existence of other, as-yet unidentified APCs also able to prime T cells. To identify those APCs, we used adenoviral (rAd) vectors, which do not infect DCs but selectively accumulate in CD169 + macrophages (MPs). In mice that lack DCs, infection of CD169 + MPs was sufficient to prime CTLs specific for all epitopes tested. In contrast, CTL responses relying exclusively on cross-presenting DCs were biased to selected strong MHC I-binding peptides only. When both DCs and MPs were absent, no CTL responses could be elicited. Therefore, CD169+ MPs can be considered APCs that significantly contribute to CTL responses. + T cells (4). In contrast, MPs are considered cells of the innate immune system that limit pathogen spread by phagocytosis and degradation. They are positioned at strategically important entry points, such as the subcapsular sinus of lymph nodes and the marginal zone and red pulp areas of the spleen, where they capture and filter pathogens (5). Along with producing type I IFN (6), CD169+ marginal zone metallophilic MPs allow restricted replication of captured virus to increase the local availability of viral Ag (7) for transfer to B cells for humoral responses (8) and to DCs for cross-priming of CD8 + T cells (9). Participation of MPs in T-cell priming is thought to be restricted to Ag acquisition, amplification, and redistribution.Analyzing MP functions in vivo is difficult; genetic models, such as the CD11c-diphteria toxin (DT) receptor (DTR) transgenic mice (2), cannot make the distinction, because both DCs and monocytes/MPs are equally deleted on DT treatment (10, 11). Recent results with mice lacking CD169 + MPs suggest the role of these MPs in cross-priming of CD8 + T cells specific for the cell-associated model Ag ovalbumin (OVA) in lymph nodes (12). These findings are in contrast with another study in which MPs were unable to cross-present soluble OVA protein in mice lacking DCs (11). Although cross-presentation by DCs is now thought to allow priming of cytotoxic T-cell responses to exogenous tumor or viral Ag (4), the importance of cross-presentation in vivo has been a matter of intense debate over the last decade (13). Although this debate remains unresolved, cross-presentation has been considered then and now to be an exception rather than a rule (14,15).To determine the roles of MPs and DCs, we used a mouse model that specifically lacks DCs but conserves all subpopulations of MPs. We show that the presence of directly infected CD169 + MPs is sufficient to prime CTLs, including those recognizing epitopes not covered by cross-presenting DCs. In contrast, when only DCs cross-prime CTLs without the participation of MPs, the CTL repertoire is incomplete and biased toward the few peptides that interact strongly with MHCI. Our data demonstrate a division of labor between CD169 + MPs and DCs in situati...

DCs very potently activate CD8+ T cells specific for viral peptides bound to MHC class I molecules. However, many viruses have evolved immune evasion mechanisms, which inactivate infected DCs and might reduce priming of T cells. Then MHC class I cross-presentation of exogenous viral Ag by non-infected DCs may become crucial to assure CD8+ T cell responses. Although many vital functions of infected DCs are inhibited in vitro by many different viruses, the contributions of cross-presentation to T cell immunity when confronted with viral immune inactivation in vivo has not been demonstrated up to now, and remains controversial. Here we show that priming of Herpes Simplex Virus (HSV)-, but not murine cytomegalovirus (mCMV)-specific CD8+ T cells was severely reduced in mice with a DC-specific cross-presentation deficiency. In contrast, while CD8+ T cell responses to mutant HSV, which lacks crucial inhibitory genes, also depended on CD8α+ DCs, they were independent of cross-presentation. Therefore HSV-specific CTL-responses entirely depend on the CD8α+ DC subset, which present via direct or cross-presentation mechanisms depending on the immune evasion equipment of virus. Our data establish the contribution of cross-presentation to counteract viral immune evasion mechanisms in some, but not all viruses.

Apoptotic cell death of Dendritic cells (DCs) is critical for immune homeostasis. Although intrinsic mechanisms controlling DC death have not been fully characterized up to now, experimentally enforced inhibition of DC-death causes various autoimmune diseases in model systems. We have generated mice deficient for Protein Phosphatase with EF-Hands 2 (Ppef2), which is selectively expressed in CD8 + DCs, but not in other related DC subtypes such as tissue CD103 + DCs. Ppef2 is down-regulated rapidly upon maturation of DCs by toll-like receptor stimuli, but not upon triggering of CD40. Ppef2-deficient CD8 + DCs accumulate the pro-apoptotic Bcl-2-like protein 11 (Bim) and show increased apoptosis and reduced competitve repopulation capacities. Furthermore, Ppef2 −/− CD8 + DCs have strongly diminished antigen presentation capacities in vivo , as CD8 + T cells primed by Ppef2 −/− CD8 + DCs undergo reduced expansion. In conclusion, our data suggests that Ppef2 is crucial to support survival of immature CD8 + DCs, while Ppef2 down-regulation during DC-maturation limits T cell responses.

Apoptotic cell death of Dendritic cells (DCs) is critical for immune homeostasis. Although intrinsic mechanisms controlling DC death have not been fully characterized up to now, experimentally enforced inhibition of DC-death causes various autoimmune diseases in model systems. We have generated mice deficient for Protein Phosphatase with EF-Hands 2 (Ppef2), which is selectively expressed in CD8 + DCs, but not in other related DC subtypes such as tissue CD103 + DCs. Ppef2 is down-regulated rapidly upon maturation of DCs by toll-like receptor stimuli, but not upon triggering of CD40. Ppef2-deficient CD8 + DCs accumulate the pro-apoptotic Bcl-2-like protein 11 (Bim) and show increased apoptosis and reduced competitve repopulation capacities. Furthermore, Ppef2 -/-CD8 + DCs have strongly diminished antigen presentation capacities in vivo, as CD8 + T cells primed by Ppef2 -/-CD8 + DCs undergo reduced expansion. In conclusion, our data suggests that Ppef2 is crucial to support survival of immature CD8 + DCs, while Ppef2 down-regulation during DC-maturation limits T cell responses.