1. Integrins and T cell movement in non-lymphoid tissue
Very little is known about the movement of T cells in non-lymphoid sites. Collagen matrices have been utilized in vitro to demonstrate that T cells exhibit a high rate of motility and form short-lived interactions with DCs.144
This pattern of motility is strikingly similar to the behavior of naïve T cells in LNs, as observed with two-photon imaging46;62
. This suggests that T cells in non-lymphoid tissue may also exhibit high rates of basal motility. In addition, the presence of collagen may promote migratory behavior, as exposure of lymphoid cells to collagen in vitro stimulates cell migration.145
In vitro studies have provided conflicting results on the role of integrins in T cell motility through collagen matrices.146;147
In one study, antibodies specific for β1, β2, β3 and αV integrins did not alter T cell movement in collagen gels.148
Genetic ablation of all integrins expressed on wild-type mouse DCs has recently been shown to have no effect on DC migration in a three-dimensional matrix in vitro and in the ear dermis in vivo.149
The dispensability of integrins for DC movement was explained by a “flowing and squeezing” model, where the protrusive flow of the actin cytoskeleton drives the cell forward and a myosin-II contractile module propels the trailing nucleus through confined ECM spaces. These findings support the view of integrins as immobilizers rather than locomotive molecules on leukocytes. This predicts a role for integrins in tissue retention instead of migration. The phenotype or function of T cells from these global integrin-deficient mice was not reported.
In a murine CD4 T cell line, septins have recently been revealed to provide cortical rigidity and the absence of septin 7 results in defective motility.150
Septins are hypothesized to promote a stable platform from which actomycin filaments can apply force, resulting in forward protrusion of the leading edge. The role of integrins in this process remains unclear, although motility speed on ICAM-1 coated coverslips under shear flow was found to be septin-dependent.
2. Regulation of integrin expression and function by the non-lymphoid tissue microenvironment
Tissue resident effector/memory T cell exhibit distinct functions, such as the ability to rapidly produce cytokine, that are not seen in their lymphoid resident counterparts. The expression of specific integrins is also known to be altered following entrance into non-lymphoid sites. These phenotypic changes following T cell entry are dependent on factors in the non-lymphoid tissue microenvironment.151
The first place these phenotypic changes can be imparted upon T cells is during the transmigration process though the endothelium. αLβ2 integrin-dependent interactions between CD4 T cells and endothelial cells in vitro result in activation of the transcription factor AP-1 without sustained NF-κB activation.152
Transmigrated T cells also upregulate αL and α4 integrin, are more migratory and are hyperresponsive to antigenic stimulation. Several other reports have also demonstrated enhanced T cell sensitivity to antigen following transmigration through endothelium.153;154
Once in non-lymphoid sites, the microenvironmental milieu may further alter the function and phenotype of the tissue resident T cell. Following LCMV infection, splenic and intraepithelial lymphocyte (IEL) memory CD8 T cells demonstrate different function and phenotype. For example, the IELs express dramatically higher granzyme B, a marker of cytolytic function, and higher αEβ7 integrin expression. However, in vivo restimulation of αEβ7 high IELs following adoptive transfer into secondary recipient mice resulted in tissue-specific development of function and phenotype. Restimulated T cells harvested from the spleen were αEβ7 integrin low, while restimulated T cells harvested from the intestine were αEβ7 integrin high.155
Reversible alterations in memory T cell expression of α4 integrin in the peritoneum and αL integrin in the lung have also been described.156;157
This suggests that T cell function and integrin expression is dependent upon tonic/continuous environmental factors present in the particular tissue.
Integrins likely also transduce signals to T cells upon engagement with ligands in the microenvironment. In non-lymphoid tissue, T cell interactions with ECM proteins such as fibronectin, laminin and collagen may provide signals that facilitate T cell activation responses. This hypothesis is supported by many studies demonstrating that β1 integrin-mediated interactions of T cells with ECM proteins can enhance TCR signaling.158
3. Integrin-mediated retention of T cells in non-lymphoid tissue
The gasterointestinal tract, skin, and lungs are in constant interaction with the environment. Therefore, the maintenance of T cells in a non-lymphoid site is of great importance for sustaining protective immunity as body barriers. The epithelium of these tissues provides the first layer of defense against pathogen invasion. Situated within and underneath the epithelial layers of these organs reside immune cells poised to recognize and destroy foreign organisms. To maintain such surveillance functions, cells need to be retained and positioned properly within each of these sites. Integrins implicated in retaining T cells in non-lymphoid tissue include the αEβ7 integrin, which binds to E-cadherin, and the collagen-binding integrins α1β1 (VLA-1) and α2β1 (VLA-2).
αEβ7 integrin was originally identified to be expressed by the vast majority of IELs and nearly 50% of T cells in the lamina propria of the small intestine.159–161
Notably, αEβ7 integrin is also expressed on subsets of both murine and human CD4 regulatory T cells.162;163
Unlike the other member of the β7 integrin subfamily, α4β7, αEβ7 integrin is not thought to be involved in the homing of T cells to the intestine and it does not interact with MAdCAM-1.164;165
E-cadherin is the only known ligand for αEβ7 integrin and it is expressed on the lateral and basolateral surfaces of epithelial cells.166
Naïve murine CD8 T cells also express αEβ7 integrin but this expression is quickly lost after activation.155
Interestingly, high level expression of αEβ7 integrin is subsequently induced upon entry into the small intestine in a TGF-β dependent fashion.167;168
The entry of α4β7-high T cells into the TGF-β rich environment of the intestine is reported to result in a switch from α4β7 expression to αEβ7 expression (). The exact mechanism for this switch is currently unknown, but Smad7 transgenic mice, which have impaired TGF-β signaling, are unable to induce αEβ7 expression.169
These mice have reduced IELs similar to the αE integrin-deficient mice.170
Signaling from the CCR9 chemokine receptor upon engagement with CCL25 may be critical to enhancing αEβ7 expression and promoting adhesion to E-cadherin as well.171;172
Integrin expression induction and function in non-lymphoid tissue
The mechanism by which IELs are maintained in the intestine is still unclear. Almost all IELs express αEβ7 and αE integrin knockout mice have a reduced number of IELs.170
However, the severity of this phenotype does have some mouse strain dependence. IELs also express the α1β1 and α4β1 integrins, which have been proposed to mediate IEL binding to type IV collagen173
and intestinal mesenchymal cells,174
respectively. Although there is a 50% reduction in IELs in α1 integrin knockout mice,175
no reduction in IEL numbers was reported in β1 integrin-deficient bone marrow chimeric mice.176
The reason for this discrepancy may be due to mouse strain differences and/or compensation by other integrins or adhesion molecules.
The brain microenvironment has also been demonstrated to induce αEβ7 expression on tumor-reactive CD8 T cells, potentially enhancing their retention in the brain ().177
Expression of αEβ7 integrin on CD8 T cells may play a more general role in promoting CD8 T cell function. CD8 T cells expressing αEβ7 integrin can kill E-cadherin positive cells and this killing is inhibited by an anti-αE integrin antibody.178
In addition, αEβ7 integrin expressed on cytotoxic CD8 T cells is recruited to the IS during interaction with E-cadherin positive tumor cells and is required for cytotoxic granule polarization and release.179
TGF-β in the tumor microenvironment may play an important role in regulating α4β7 expression on tumor-resident CD8 T cells.
The α1β1 (VLA-1) and α2β1 (VLA-2) integrins were initially discovered as antigens that were induced after long-term in vitro activation of human T cells.180–182
Both α1β1 and α2β1 mediate cell adhesion to collagen. α1β1 mediates adhesion to type IV collagen, while α2β1 mediates adhesion to type I collagen. As naïve T cells do not bind to collagen, even after acute stimulation signals that promote integrin-dependent adhesion to ICAM-1, VCAM-1 and fibronectin,183
the regulated expression of α1β1 and α2β1 on effector/memory T cells may be critical to promoting effective T cell adhesion to collagen found in non-lymphoid tissue. Initial studies using integrin blocking antibodies and α1 integrin-deficient mice clearly highlighted a role for collagen-binding integrins in various inflammatory models, as well as in a mouse model of arthritis.184
Subsequent studies in a variety of model systems have suggested multiple functions in non-lymphoid tissue for α1β1 and α2β1 expressed on activated T cells.
In the lung, α1β1 integrin enables both retention and survival of influenza specific CD8 T cells.185
Although α1β1 integrin is not required for T cell entry into the lung, many more T cells in the lung are α1β1 integrin-positive compared to activated T cells in the spleen. This suggests that α1β1 integrin expression may be specifically induced on CD8 T cells after their entry into the lung. α1β1 integrin-positive cells had reduced levels of apoptosis compared to the α1β1 integrin-negative population in the lung. Other work has supported this role for α1β1 integrin in promoting the survival of lung-resident CD8 T cells, potentially via signaling mechanisms that synergize with TNF Receptor II.186
Antibody-mediated inhibition of α1β1 integrin resulted in both a loss of cells from the lungs and reduced ability to protect against a secondary challenge.185
Mice deficient in α1 integrin expression also exhibit enhanced susceptibility to secondary virus challenge. α1β1 integrin-positive CD8 T cells isolated from the lung produce effector cytokines, such as IFN-γ and TNFα. CD4 T cells in the lung express higher levels of α2β1 integrin than CD8 T cells, and this is associated with CD4 T cell localization to interstitial spaces in the lung that express type I collagen. In vitro analysis of these CD4 T cells suggest a role for α2β1 in enhancing IFN-γ production.187
Thus, these studies suggest that collagen-binding integrins promote T cell survival, retention, and effector function in the lung parachyma, thus enhancing protective immunity.
α1β1 has also been shown to retain pathogenic type 1 effector T cells in the epidermis in a xenoplant model of psoriasis.188
In this model, α1 integrin-specific blocking antibodies prevented the development of psoriasis by preventing the accumulation of these cells in the epidermis. These intraepidermal α1β1 integrin-positive T cells were also shown to co-express αEβ7 integrin. The lack of expression of α1β1 on dermal T cells suggests that α1β1 integrin may be upregulated during passage through the collagen IV rich demo-epidermal basement membrane. Entry into the E-cadherin rich epidermis might also promote upregulation of αEβ7, which would facilitate epidermal localization of CD8 T cells in the skin. In addition, the αEβ7 integrin inducing molecule, TGF-β, is found in high concentration in areas underlying psoriatic lesions.189
Overall, there is emerging evidence that the microenvironment in non-lymphoid tissue can both modulate integrin expression on T cells and provide adhesive ligands that promote T cell retention, survival and effector function. Integrins such as αEβ7, α1β1 and α2β1 have been proposed to play unique roles in promoting T cell retention in non-lymphoid tissue. However, it is known that the presence of antigen is important for the retention of effector/memory T cells in non-lymphoid sites.190
In addition, MHC class II-positive DCs have recently been reported to migrate to non-lymphoid tissue and interact with memory CD4 T cells.191
Thus, TCR-mediated signals that activate integrins may allow multiple integrins expressed on effector T cells to promote retention in tissue via adhesion to APCs or ECM proteins.