CCR5 is expressed on a variety of hematopoietic cell types including DCs, macrophages, and Th1-type T cells (23
). However, it remains unclear whether CCR5 expression by these cell types has a biological significance during in vivo immune responses. Here, we provide a detailed characterization of innate and adaptive immune responses to L. monocytogenes
in CCR5-deficient hosts. In our study, the putative roles of CCR5 in both cellular recruitment and activation under inflammatory conditions were examined. We demonstrate that recruitment of macrophages, DCs, and CD8 T cells during both primary and secondary immune responses to L. monocytogenes
occurs normally in CCR5−/−
mice. Furthermore, CCR5 is not required for the induction of TNF, IL-12, and IFN-γ during innate responses, activation of bactericidal mechanisms, and priming of Listeria
-specific CD8 T cells.
Immediately following introduction of the infectious agent into the host environment, inflammatory events take place which are characterized by the establishment of chemotactic gradients redirecting normally circulating cells into the sites of injury. In vivo expression of MIP-1α in the spleens of L. monocytogenes
-infected mice and in vitro expression of MIP-1α and MIP-1β by Listeria
-infected bone marrow-derived macrophages has been reported (10
). In vitro, CCR5 ligation induces the migration of macrophages, immature DCs, and activated and memory Th1 T cells (23
). In vivo, injection of soluble T. gondii
antigens induces CCR5-dependent migration of CD8α+
DCs from the red pulp and marginal-zone areas of the spleen to T-cell-containing areas of the white pulp (1
). During infection with Leishmania donovanii
, the granulomatous response in the liver is impaired in the absence of CCR5, suggesting the presence of abnormalities in the cell trafficking (34
). In contrast, we observed normal migration of DCs and macrophages and unimpaired formation of bacterium-containing lesions in spleens and livers during the anti-listerial immune response in CCR5-deficient mice.
It is intriguing that despite abundant CCR5 expression in the infected tissues, CCR5 deficiency had no apparent effect during the immune response to L. monocytogenes.
One difficulty in understanding the roles of individual chemokines and chemokine receptors stems from their redundancy: many receptors have the same chemokine ligands, and many chemokines bind several chemokine receptors. Only one CCR5 ligand, MIP-1β, is unique to this receptor, while MIP-1α, RANTES, and MCP are shared with CCR1, CCR2, CCR3, and CCR4 (45
). During L. monocytogenes
infection, other receptors might be utilized by MIP-1α and RANTES in the absence of CCR5 in vivo, an issue that could be addressed only by generating mice with multiple chemokine receptor deficiencies.
In this respect, it is notable that mice lacking CCR2 have pronounced deficiencies in monocyte recruitment in vivo and are highly susceptible to L. monocytogenes
infection. CCR2 is the sole receptor for its major ligand, MCP-1 (45
). Thus, redundancy in the use of chemokine receptors might be a correlate of how well the particular chemokine deficiency will be tolerated during the inflammation. Additionally, the relative importance of individual chemokine receptors during inflammatory reactions might depend strongly on the preferential site for parasite multiplication. Successful immunity to C. neoformans
requires CCR5 expression (19
). However, the susceptibility is associated with the defective recruitment of leukocytes to the brains of C. neoformans
-infected mice while cellular migration to the lungs appears intact, suggesting the existence of tissue-specific requirements for CCR5.
In addition to inducing cellular migration, other biological functions have been ascribed to CCR5 and its ligands. In vitro, MIP-1α induces the secretion of TNF, IL-6, and IL-1 by peritoneal macrophages, suggesting a role for this chemokine in the direct activation of cells (14
). In line with these findings, CCR5-deficient peritoneal macrophages were reported to have partial defects in the production of IL-6 and IL-1 in response to MIP-1α (44
). In vivo, administration of a tachyzoite antigen extraxt (STAg) to mice induced strong CCR5-driven IL-12 production by CD8α+
). Recently, it was shown that TNF and IL-12 secretion by L. monocytogenes
-infected bone marrow-derived macrophages in response to IFN-γ in vitro is enhanced by treatment with MIP-1α, MIP-1β, and RANTES, suggesting synergism between Th1-type cytokines and CC chemokines (11
). However, the latter in vitro findings do not appear to correlate with our in vivo observations, since there was no reduction in the levels of TNF, IL-12, or IFN-γ during primary infection of CCR5-deficient mice with L. monocytogenes.
In agreement with normal cytokine secretion, expression of iNOS was also intact, indicating that the induction of bactericidal functions was unimpaired. Interestingly, T. gondii
cyclophilin has been shown to bind CCR5, mimicking its chemokine ligands and triggering cytokine secretion and DC migration, suggesting that some pathogens might actively exploit the expression of chemokine receptors on inflammatory cells (2
In addition to its expression on cells of the innate immune system, CCR5 expression on effector and memory T cells has been reported (16
). In vitro, MIP-1α, MIP-1β, and RANTES enhance antigen-specific T-cell proliferation and IL-2 production (41
). All CCR5 ligands were demonstrated to be chemotactic for Th1-type T cells, and MIP-1α was shown to induce the migration of activated CD8 T cells (40
). Lymphocytes migrating to the liver in response to hepatitis C virus infection express CCR5, and it was suggested that tissue infiltration by lymphocytes correlates with the high level of CCR5 expression (36
). However, most of our understanding of the role of CCR5 expression on T cells comes from in vitro chemotaxis experiments and the phenotypic analysis of peripheral lymphocytes. Our study directly addressed the role of CCR5 in memory T-cell migration to the sites of inflammation during in vivo bacterial infection. Recall CD8 T-cell responses in the spleens and livers of L. monocytogenes
mice were comparable to those observed in wild-type control mice, suggesting that CCR5 expression by memory T cells is not essential for their migration. Other chemokine receptors such as CCR2 and CXCR3 may recruit memory T cells to the sites of inflammation in the absence of CCR5. Additionally, the presence of long-lived memory cells in the nonlymphoid tissues has been shown (24
), suggesting that the need for chemokine-mediated cellular recruitment might be bypassed in these tissues by directly activating resident cells.
CCR5 is a major coreceptor used by M-tropic strain of HIV-1 (3
). Individuals homozygous for the mutant allele, CCR5-Δ32, are highly resistant to HIV infection, and heterozygotes show delayed disease progression (34
). CCR5 ligands can prevent the entry of HIV-1 into cells, suggesting that strategies limiting the normal availability of surface CCR5 to viral gp120 are highly promising in reducing HIV infectivity (7
). However, the expression of CCR5 on cells critical for both innate and adaptive immune responses raises the question of the impact of CCR5 blockade on the immune responses to other pathogens. To date, the in vivo role of CCR5 has been characterized only with respect to fungal (C. neoformans
) and protozoan (T. gondii
) pathogens (1
). Our study provides the first comprehensive examination of the in vivo immune responses to an intracellular bacterial pathogen in the absence of CCR5 and suggests that CCR5 is dispensable for both migration and activation of cells during primary and secondary immune responses to L. monocytogenes
. Although the susceptibility of CCR5-deficient mice to other bacterial pathogens remains to be tested, our data correlate with the finding that CCR5-deficient humans are not detectably more susceptible to bacterial infections.