Intracellular Pathogens and CD8+ Dendritic Cells: Dangerous Liaisons
Authors: Boris Reizis
Journal: Immunity (2011)

CD8+ dendritic cells comprise a distinct cell type whose function is unclear. In this issue of Immunity, Mashayekhi et al., 2011Mashayekhi M. Sandau M.M. Dunay I.R. Frickel E.M. Khan A. Goldszmid R.S. Sher A. Ploegh H.L. Murphy T.L. Sibley L.D. Murphy K.M. Immunity. 2011; 35 (this issue): 249-259Abstract Full Text Full Text PDF PubMed Scopus (285) Google Scholar show these cells are essential for protection against the parasite Toxoplasma, but Edelson et al., 2011Edelson B.T. Bradstreet T.R. Hildner J. Carrero J.A. Frederick K.E. KC W. Belizaire R. Aoshi T. Schreiber R.D. Miller M.J. et al.Immunity. 2011; 35 (this issue): 236-248Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar show they are hijacked by Listeria during initial spreading. CD8+ dendritic cells comprise a distinct cell type whose function is unclear. In this issue of Immunity, Mashayekhi et al., 2011Mashayekhi M. Sandau M.M. Dunay I.R. Frickel E.M. Khan A. Goldszmid R.S. Sher A. Ploegh H.L. Murphy T.L. Sibley L.D. Murphy K.M. Immunity. 2011; 35 (this issue): 249-259Abstract Full Text Full Text PDF PubMed Scopus (285) Google Scholar show these cells are essential for protection against the parasite Toxoplasma, but Edelson et al., 2011Edelson B.T. Bradstreet T.R. Hildner J. Carrero J.A. Frederick K.E. KC W. Belizaire R. Aoshi T. Schreiber R.D. Miller M.J. et al.Immunity. 2011; 35 (this issue): 236-248Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar show they are hijacked by Listeria during initial spreading. Dendritic cells (DCs) are a distinct lineage of mononuclear phagocytes that excel at pathogen sensing, cytokine secretion, and antigen presentation. The classical DCs comprise two distinct subsets, distinguished in the mouse by the expression of CD8α (Shortman and Heath, 2010Shortman K. Heath W.R. Immunol. Rev. 2010; 234: 18-31Crossref PubMed Scopus (391) Google Scholar). This subset dichotomy exists not only in lymphoid organs but also in tissues, in which CD103+ DCs represent a genetic and functional equivalent of CD8+ DCs. Notably, CD8+ and CD103+ DCs are found in relatively low numbers in vivo (0.1%–0.2% of murine splenocytes), to the dismay of researchers who study them. A DC subset similar to CD8+ DCs has been identified in humans, with this conservation probably reflecting an essential role in immunity. An overwhelming body of evidence suggests that CD8+ DCs are particularly efficient at cross-presentation, i.e., the presentation of exogenously acquired antigens on MHC class I molecules to CD8+ T cells. Although this property is important in certain settings such as immune responses to tumors and viruses, a broader role of CD8+ DCs in immunity to infections remains poorly understood. As any good defense system, the immune system has multiple layers of protection and redundancy, often frustrating our attempts to define the roles of individual cell types. Luckily, these attempts have been aided by molecular insights into the development of immune cell lineages, including DC subsets. In 2008, Murphy and colleagues identified the transcription factor Batf3 as a specific regulator of CD8+ DC development (Hildner et al., 2008Hildner K. Edelson B.T. Purtha W.E. Diamond M. Matsushita H. Kohyama M. Calderon B. Schraml B.U. Unanue E.R. Diamond M.S. et al.Science. 2008; 322: 1097-1100Crossref PubMed Scopus (1347) Google Scholar). Batf3 is expressed in both CD8+ and CD8- DC subsets, yet the deletion of Batf3 caused a specific loss of CD8+ DCs. Although the molecular mechanism of subset-specific Batf3 activity remains to be characterized, Batf3-deficient mice provide an elegant system for addressing the function of CD8+ DCs in vivo. As expected, these mice are deficient in cross-presentation and tumor cell rejection and show impaired priming of virus-specific CD8+ T cells. Toxoplasma gondii is an intracellular protozoan parasite that elicits strong immune response involving both innate and adaptive mechanisms. Interferon gamma (IFN-γ) secreted by T cells and other cell types such as NK cells is critical for Toxoplasma clearance, and interleukin-12 (IL-12) is an essential inducer of IFN-γ production. Reis e Sousa et al., 1997Reis e Sousa C. Hieny S. Scharton-Kersten T. Jankovic D. Charest H. Germain R.N. Sher A. J. Exp. Med. 1997; 186: 1819-1829Crossref PubMed Scopus (761) Google Scholar first demonstrated that DCs represent a primary source of IL-12 in response to Toxoplasma infection. Indeed, DC depletion abrogated IL-12 and IFNγ production and made mice highly susceptible to Toxoplasma (Liu et al., 2006Liu C.H. Fan Y.T. Dias A. Esper L. Corn R.A. Bafica A. Machado F.S. Aliberti J. J. Immunol. 2006; 177: 31-35Crossref PubMed Scopus (151) Google Scholar). More recently, blockade of Toll-like receptor (TLR) and interleukin-1 signaling in DCs but not in macrophages or neutrophils was shown to impair IL-12 production and increase susceptibility to Toxoplasma (Hou et al., 2011Hou B. Benson A. Kuzmich L. DeFranco A.L. Yarovinsky F. Proc. Natl. Acad. Sci. USA. 2011; 108: 278-283Crossref PubMed Scopus (88) Google Scholar). These results collectively suggest that DCs directly sense Toxoplasma through TLRs and secrete IL-12 to initiate the IFN-γ-dominated protective immunity. However, the precise DC subset(s) responsible for IL-12 secretion remained moot. In this issue of Immunity, Mashayekhi et al., 2011Mashayekhi M. Sandau M.M. Dunay I.R. Frickel E.M. Khan A. Goldszmid R.S. Sher A. Ploegh H.L. Murphy T.L. Sibley L.D. Murphy K.M. Immunity. 2011; 35 (this issue): 249-259Abstract Full Text Full Text PDF PubMed Scopus (285) Google Scholar now demonstrate that Batf3-deficient mice are highly susceptible to Toxoplasma, show greatly diminished IL-12 and IFN-γ production, and fail to expand Toxoplasma-specific CD8+ T cells (Figure 1). As in the study by Hou et al., IL-12 administration substantially rescued these defects, suggesting that IL-12 production rather than antigen presentation by CD8+ DCs is essential for protection. A clever reconstitution experiment generated chimeric mice in which CD8+ DCs were present but unable to produce IL-12, and these chimeras showed the same susceptibility to infection. These data clearly establish IL-12 production as a unique property of CD8+ DCs that is essential for protective immune response against Toxoplasma. In agreement with this, the study describes massive expansion of CD8+ DCs during the natural course of infection—an intriguing phenomenon that warrants future studies of its mechanism. Given the powerful IL-12 secretion and cross-presentation capacity of CD8+ DCs, an intuitive assumption would be that more of these cells would confer more protection against any intracellular pathogen. However, Wilson and colleagues reported back in 2004 that the administration of DC growth factor Flt3L increased sensitivity to obligate intracellular bacterial pathogens such as Listeria monocytogenes (Alaniz et al., 2004Alaniz R.C. Sandall S. Thomas E.K. Wilson C.B. J. Immunol. 2004; 172: 3725-3735Crossref PubMed Scopus (36) Google Scholar). These authors proposed that DCs are “sentinels without armament” that capture and present bacteria but cannot kill them; thus DC expansion increases the cellular niche for bacterial replication. Notably, Flt3L preferentially expands CD8+ DCs, implicating this subset as a potential culprit. More recently, mice with a specific expansion of CD8+ DC compartment were similarly shown to be highly susceptible to Listeria (Sathaliyawala et al., 2010Sathaliyawala T. O'Gorman W.E. Greter M. Bogunovic M. Konjufca V. Hou Z.E. Nolan G.P. Miller M.J. Merad M. Reizis B. Immunity. 2010; 33: 597-606Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar). In both studies, increased early replication of Listeria was observed, whereas the subsequent CD8+ T cell response was normal or increased. These observations suggest that CD8+ DCs may represent “Trojan horses” for the entry and/or early expansion of Listeria, albeit in artificial settings of cytokine administration or genetic manipulation. In parallel, an elegant study from the Busch group (Neuenhahn et al., 2006Neuenhahn M. Kerksiek K.M. Nauerth M. Suhre M.H. Schiemann M. Gebhardt F.E. Stemberger C. Panthel K. Schroder S. Chakraborty T. et al.Immunity. 2006; 25: 619-630Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar) showed that DC depletion abrogated Listeria infection by preventing its early spread and proliferation in the spleen (but not in the liver). Indeed, DCs were shown by multiphoton microscopy to rapidly deliver Listeria to the lymphocyte-rich splenic white pulp (Aoshi et al., 2008Aoshi T. Zinselmeyer B.H. Konjufca V. Lynch J.N. Zhang X. Koide Y. Miller M.J. Immunity. 2008; 29: 476-486Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar), in which it replicates rapidly and causes widespread apoptosis. Although Neuenhahn et al. implicated CD8+ DC subset in the initial capture and transport of Listeria, a system to specifically deplete CD8+ DCs was not available at the time. Now in the same issue of Immunity, Edelson et al., 2011Edelson B.T. Bradstreet T.R. Hildner J. Carrero J.A. Frederick K.E. KC W. Belizaire R. Aoshi T. Schreiber R.D. Miller M.J. et al.Immunity. 2011; 35 (this issue): 236-248Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar used Batf3-deficient animals to explicitly identify CD8+ DCs as a key cell type for the entry and early spread of Listeria (Edelson et al., 2011Edelson B.T. Bradstreet T.R. Hildner J. Carrero J.A. Frederick K.E. KC W. Belizaire R. Aoshi T. Schreiber R.D. Miller M.J. et al.Immunity. 2011; 35 (this issue): 236-248Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar) (Figure 1). Indeed, Batf3-deficient mice are essentially resistant to Listeria, requiring 100- to 1000-fold more bacteria for equivalent bacterial replication. In the absence of CD8+ DCs, bacteria failed to spread to the white pulp and instead were rapidly cleared by phagocytic cells. By microscopy and flow cytometry, Edelson et al., 2011Edelson B.T. Bradstreet T.R. Hildner J. Carrero J.A. Frederick K.E. KC W. Belizaire R. Aoshi T. Schreiber R.D. Miller M.J. et al.Immunity. 2011; 35 (this issue): 236-248Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar detected occasional CD8+ DCs that become infected with Listeria and carry it to the white pulp, suggesting that it is a rare but nevertheless essential event for productive infection. The study by Edelson et al., 2011Edelson B.T. Bradstreet T.R. Hildner J. Carrero J.A. Frederick K.E. KC W. Belizaire R. Aoshi T. Schreiber R.D. Miller M.J. et al.Immunity. 2011; 35 (this issue): 236-248Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar provides important additional insights into the role of CD8+ DC subset in intracellular bacterial infection. First, Batf3-deficient mice were more resistant to Listeria ivanovii, which replicates only in the liver but not in the spleen. This result provides strong genetic evidence that CD103+ DCs in the liver play an equivalent role in the transport and early replication of Listeria. Second, the absence of CD8+ DCs did not diminish protective T cell response to Listeria, particularly when adjusted for bacterial burden. Therefore, different antigen-presenting cell type(s) such as CD8- DCs or macrophages are fully capable of priming a functional T cell response to Listeria. This is perhaps not surprising, given that the antigens of intracellular Listeria may have access to the canonical endogenous presentation pathway and would not require cross-presentation. Nevertheless, these results clearly dissociate the obligatory (and detrimental) role of CD8+ DCs in the capture and transport of Listeria from their dispensable role in T cell priming. The studies by Mashayekhi et al., 2011Mashayekhi M. Sandau M.M. Dunay I.R. Frickel E.M. Khan A. Goldszmid R.S. Sher A. Ploegh H.L. Murphy T.L. Sibley L.D. Murphy K.M. Immunity. 2011; 35 (this issue): 249-259Abstract Full Text Full Text PDF PubMed Scopus (285) Google Scholar and Edelson et al., 2011Edelson B.T. Bradstreet T.R. Hildner J. Carrero J.A. Frederick K.E. KC W. Belizaire R. Aoshi T. Schreiber R.D. Miller M.J. et al.Immunity. 2011; 35 (this issue): 236-248Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar are significant in several ways. First, they provide clear genetic evidence that DCs and their subsets such as CD8+ DCs have specific nonredundant functions in “real” immune responses. Because immunologists tend to endlessly subdivide any cell type by using random surface markers, this is a deeply satisfying reality check. Second, they highlight the unique role of DCs in pathogen sensing and initiation of immune responses, as opposed to direct pathogen killing mediated by other myeloid cells. This is a clear asset in some infections such as Toxoplasma, where CD8+ DCs rapidly activate multiple cell types to contain the pathogen; however, clever pathogens such as Listeria can exploit it to facilitate their own spread. Finally, the identification of CD8+ DCs as a liability in certain infections provides a compelling reason for very low numbers of these cells in vivo. Indeed, specific cell-intrinsic mechanisms appear to restrict proliferative signals and control the number of CD8+ DCs in the steady state (Sathaliyawala et al., 2010Sathaliyawala T. O'Gorman W.E. Greter M. Bogunovic M. Konjufca V. Hou Z.E. Nolan G.P. Miller M.J. Merad M. Reizis B. Immunity. 2010; 33: 597-606Abstract Full Text Full Text PDF PubMed Scopus (124) Google Scholar). This caveat should be taken into consideration in any attempt to expand or reconstitute DCs for therapeutic purposes, e.g., as anticancer vaccines.