The interaction of invasive bacterial pathogens of the gut such as Yersinia
with the host causes a potent inflammatory response and tissue damage, leading to significant human morbidity and mortality (16
). Several lines of evidence suggest that even during the acute response to infection tempering the inflammatory response can be beneficial to the host. For example, previous work indicates that IL-6−/−
mice have a hyperinflammatory response to Y. enterocolitica
infection that results in a more rapid death of the animals and tissue pathologies that reflect exacerbated inflammatory changes (26
). These studies also demonstrated an IL-6-dependent defect in TGF-β secretion from stimulated splenocytes that correlated with an older observation that mice treated with recombinant TGF-β were more resistant to Y. enterocolitica
). TGF-β is a potent anti-inflammatory cytokine (36
); in fact, it wasn't possible to use TGF-β−/−
mice for these studies because the animals die 2 to 3 weeks after birth from systemic inflammation (43
). Altogether, these data suggest that cytokine-mediated regulation of the extent of inflammation during an invasive bacterial infection is a crucial aspect of the host response.
Cytokine-mediated immune regulation is often carried out by CD4 +
helper T cells (Th), which polarize the immune system to appropriately respond to pathogen challenge. There is strong experimental evidence supporting a central role for T cells in clearing Y. enterocolitica
infection, especially IFN-γ-producing Th-1 cells. However, in the last 10 years, a number of newly discovered CD4+
T-cell subsets (T-reg and Th-17) have emerged as key players in the pathogenesis of diseases ranging from infectious disease to autoimmune disease (37
). T-reg cells are critical to the maintenance of immune tolerance and prevention of unwanted inflammatory responses at mucosal surfaces such as the gut, and, not surprisingly, T-regs have been implicated in the pathogenesis of a variety of chronic infectious diseases (32
). Several studies have also implicated T-regs in the modulation of memory responses following an acute infection, but little information on the direct role of T-regs during the host response to an acute bacterial infection is available.
Recently, Rudensky and coworkers reported that T-regs maintain Th-17 homeostasis in the gut utilizing a STAT-3-dependent mechanism that was distinct from that used by TGF-β (21
). These studies also highlighted the interaction of different classes of T cells to maintain tolerance of the gut. It would not be unreasonable to predict that similar control mechanisms are in place to control inflammation generated in response to acute bacterial infections of the gut.
Here we report a role for T-regs during the host response to acute invasive infection of the gastrointestinal tract by Y. enterocolitica
. By 3 days postinfection there is a rapid expansion of CD4+
T cells and a 2- to 3-fold increase in TGF-β+
T cells yet there is no increase in CD8+
TGF-β-producing T cells or macrophages producing TGF-β. T-reg cells are critical for controlling dissemination to extraintestinal sites of infection based on the more rapid liver colonization and the subsequent rapid death of mice treated with anti-TGF-β or anti-CD25 antibodies. Active TGF-β transcription in the Peyer's patches, mesenteric lymph nodes, and spleen following infection is dependent on the presence of CD4+
cells, suggesting that T cells are the major source of this cytokine during infection (Fig. ). The importance of CD4+
T cells is well established as an immune control mechanism during Yersinia
infection, but, to our knowledge, this is the first report demonstrating a protective role for a CD4+
T-cell subset other than Th-1 for Yersinia
The protective role of T-regs is presumably secondary to that provided by Th-1 or Th-17 T cells given that all of the control animals in these experiments developed disease and in many cases succumbed to infection. Here, we provide evidence for the generation of CD4+ Th-17 cells following Y. enterocolitica infection that is consistent with the presence of IL-6, IL-1, and TGF-β in infected tissues. However, in a situation where an animal is deficient in TGF-β or CD25 function, bacteria disseminate to the liver and lung more rapidly and the majority of animals succumb to disease several days before the control animals. Interestingly, we show that animals treated with antibodies that block TGF-β or CD25 function have a diminished Th-17 response following Y. enterocolitica infection. These data are consistent with a T-reg response during acute infection impacting the appropriate Th-17 effector T-cell response. However these data should be interpreted with some caution as Th-17 cells express IL-2r and it is possible that anti-CD25 treatment might impact IL-17 production by blocking IL-2r on these cells. These data do demonstrate a clear Th-17 response during Y. enterocolitica infection, further expanding the known CD4+ effector responses elicited during this infection.
While CD25 can be expressed on other CD4+
effector T cells once they are activated, the kinetics of liver and lung colonization in the mice treated with anti-CD25 antibodies suggest a target T cell expressing high levels of CD25 at days 0 to 3 postinfection during a primary immune response, which is most consistent with T-regs. Interestingly, another potent anti-inflammatory cytokine produced by T-regs is IL-10, which has a controversial role in the pathogenesis of Y. enterocolitica
infection, with some studies suggesting that deficiency in IL-10 leads to resistance and others demonstrating that IL-10 deficiency has no impact on Yersinia
). The different phenotypes observed with IL-10 versus TGF-β deficiency may be reflective of distinct roles during the host response or a difference due to genetic versus antibody-mediated deficiency. Altogether, these data would suggest that T-regs, through the action of TGF-β, help to limit bacterial dissemination to the liver and lung by an unknown mechanism.
A number of recent studies have suggested that the dissemination of Yersinia
from the gut to other organs is more complex than previously appreciated, finding that Peyer's patches are not absolutely required for infection through the gut and that pools of bacteria in the gut can seed the deeper tissues (9
). Given these revelations, it is not unreasonable to predict that the host has evolved a means to temper inflammation during an acute bacterial infection as a means to diminish tissue damage and limit dissemination of the infection.
In the case of invasive bacterial pathogens of the gut, natural T-regulatory cells appear to figure prominently in this role. Studies with ovalbumin (OVA)-restricted mice suggest that a fraction of the increases in T-regs after Y. enterocolitica
infection are independent of clonal expansion and may be a response to pathogen-associated antigens recognized through the Toll-like receptors expressed on these cells (46
). Likewise, these same data would suggest that the expansion of T-regs in the Yersinia
-infected C57BL/6 mice is predominantly due to antigen-dependent expansion of these cells. Currently it is not clear if these cells expand due to a pathogen-associated antigen or in response to a self antigen released as a result of the severe tissue damage caused by these infections.
Altogether, these studies reveal a previously unrecognized role for T-regs in the acute phase of the host response to invasive bacterial pathogens of the gut. Although T-regs are unlikely to be directly involved in antimicrobial activities in a manner analogous to Th-1 cells, they likely control inflammation, thus limiting tissue damage and direct access to the circulation, thereby indirectly impacting the dissemination of bacteria. The robust colonization of the livers by day 5 postinfection in mice treated with anti-TGF-β or anti-CD25 antibodies suggests that these mice develop bacteremia more rapidly than control animals. Additionally, during acute infections, T-regs may play an important role in regulating immune homeostasis, limiting the extent of inflammation and/or assisting in the return to basal status once infection has been resolved. These data supply further evidence for mechanisms of immune control of acute inflammation during invasive bacterial infection.