Here we demonstrate an essential role for IEL in proinflammatory intestinal pathology induced by infection with T. gondii. Damage to the intestinal mucosa was furthermore dependent upon ability of the cells to produce IFN-γ. In addition, emergence of pathogenic IEL during infection required expression of CCR2. Consequently, mice lacking CCR2 were protected from development of ileal inflammation. CD4+ T lymphocytes have previously been implicated in Toxoplasma-induced intestinal pathology, but this study now shows that the IEL compartment, mostly composed of CD8+ T cells, also possess pathogenic activity during infection.
Lack of CCR2 in Toxoplasma
-infected mice was recently associated with failure to recruit antimicrobe inflammatory monocytes and increased pathology in several tissues, including the intestine 17
. In the present study we also found that CCR2−/−
mice are more susceptible to infection using a low parasite innoculum, as measured by increased mortality and significantly higher parasite burdens in the surviving mice. However, following high dose infection we found that deletion of CCR2 mediates protection against pathology in the small intestine. In agreement with us, another recent study found that CCR2−/−
mice are resistant to Toxoplasma
-induced damage to the gut 18
. Issues of parasite dose may underlie differences reported in these studies regarding parasite-induced pathology in CCR2−/−
mice. Low infection inoculums employed by Dunay et al. 17
may trigger CCR2-dependent inflammatory monocytes whose absence leads to uncontrolled parasite replication associated with tissue destruction at the site of infection. Under higher infection conditions, CCR2 appears to be important in recruitment and activation of IEL that cause proinflammatory pathology in the intestine.
The CCR2-dependent IEL identified in this study were a mixed population. While the cells uniformly expressed CD103, approximately half were positive for CD11c, a molecule known to be expressed on activated T cells in the intestinal mucosa 23
. Our data show that both CD11c+
IEL induce damage, but further studies are needed to determine if this is an activity mediated by αβ or γδ T lymphocytes that are present in both populations.
The CD103 molecule, also known as αE, forms a heterodimer with the integrin β7 subunit. Binding of CD103/β7 to epithelial cell E-cadherin is important in homing and retention of lymphocytes and dendritic cells to the intestinal mucosa 22
. There is evidence that CD103 is involved in intestinal graft-versus-host disease pathology and TNP-OVA-induced colitis in IL-2-deficient mice 26, 27
. Other studies suggest that CD103+
T cells regulate pathology in TNF-mediated experimental colitis, and there is evidence that CD103+
DC control disease in T cell transfer models of colitis 28, 29
. Thus, whether CD103 is associated with inflammation or prevention of pathology most likely depends upon the effector function of the cells expressing this intestinal homing molecule.
Although we found that IEL contribute to pathology in the gut during infection, this compartment has been linked to protection and maintenance of homeostasis during T. gondii
infection 12, 24, 25
. While LP CD4+
T cells were found to synergize with intestinal epithelial cells for proinflammatory cytokine production, IEL secretion of TGF-β down-regulated the response, and in vivo transfer studies suggested a role for this anti-inflammatory cytokine in preventing parasite-induced ileitis 12, 20, 25
. Furthermore, γδ T cells have been associated with epithelial barrier function during Toxoplasma
. Other studies have shown that adoptive transfer of IEL protects against infection and induces long-term immunity against Toxoplasma 24, 31, 32
. Consistent with our studies, IEL effector activity in these cases was associated with production of IFN-γ 24
The divergence between the protective function of the IEL compartment reported previously 24
and the clear pathologic activity reported here may be a consequence of differences in parasite doses employed in each case. Whereas we isolated IEL from animals undergoing high dose infection, earlier studies employed IEL from animals undergoing low dose infection. We hypothesize that exposure to a low parasite dose elicits IEL activity that mediates protection against inflammatory pathology, and that high parasite doses triggers a switch to IEL that mediate inflammatory pathology in the intestine.
Based upon previous studies in animal models, IBD pathogenesis has been attributed to the activity of lamina propria CD4+
cells responding to gut flora 33
. Here, we show that IEL induce similar pathology. Because the overwhelming majority of these cells express the CD8 molecule, it is likely that this IEL subset mediates disease. In particular, the CD11c+
IEL that are approximately 98% positive for CD8 transfer pathology upon inoculation into CCR2−/−
mice. However, we cannot completely exclude a role for CD4+
T lymphocytes because these cells comprise approximately 20% of the CD103+
population that also mediates damage.
There is increasing evidence that CD8+
T cells may trigger pathogenic CD4+
responses during IBD, possibly by damaging the epithelium and allowing access of luminal bacteria or by releasing activating cytokines 33
. In a hapten-induced colitis model, hapten-sensitized CD8+
T cells were identified as the earliest initiators of infection 30
,. In another study, transgenic CD8 T cells specific for influenza hemagglutinin A (HA) that had developed in the absence of cognate antigen were able to mediate severe intestinal destruction after transfer into transgenic mice expressing an HA transgene in the intestinal epithelial compartment 31
. The results of the present study reinforce and extend the hypothesis that pathogenic CD8+
T cells are initiators of intestinal pathology. Importantly, our study provides evidence that IEL themselves mediate ileal damage triggered by microbial infection. Furthermore, our results reveal for the first time chemokine receptor CCR2 as a key player in this pathology.
Unraveling the roles of IEL in the pathogenesis of intestinal inflammation is a complex task. The IEL compartment is diverse, consisting of T cells expressing αβ and γδ T cell receptors 34
. Most IEL express the CD8 molecule, yet of these, some are generated independently of classical major histocompatibility complex (MHC) class I molecules 35, 36
. Some IEL are generated in the thymus, but other subpopulations are believed to derive from cryptopatches in the intestinal mucosa 22
. By examining expression of chemokine receptors and T cell receptors, as well as the need for MHC in generating the cells, it should be possible to elucidate requirements for local recruitment and expansion of protective or pathogenic IEL. In turn, this can be expected to shed light on pathogenesis of IBD.