Collagen distribution and expression of collagen-binding alpha1beta1 (VLA-1) and alpha2beta1 (VLA-2) integrins on CD4 and CD8 T cells during influenza infection

Most viral infections occur in extralymphoid tissues, yet the mechanisms that regulate lymphocytes in these environments are poorly understood. One feature common to many extralymphoid environments is an abundance of extracellular matrix. We have studied the expression of two members of the beta(1) integrin family of collagen-binding receptors, alpha(1)beta(1) and alpha(2)beta(1) (CD49a, VLA-1 and CD49b, VLA-2, respectively), on CD4 and CD8 T cells during the response to influenza infection in the lung. Flow cytometry showed that whereas T cells infiltrating the lung and airways can express both CD49a and CD49b, CD49a expression was most strongly associated with the CD8+ subset. Conversely, though fewer CD4+ T cells expressed CD49a, most CD4+ cells in the lung tissue or airways expressed CD49b. This reciprocal pattern suggested that CD4 and CD8 T cells might localize differently within the lung tissue and this was supported by immunofluorescent analysis. CD8+ cells tended to localize in close proximity to the collagen IV-rich basement membranes of either the airways or blood vessels, whereas CD4+ cells tended to localize in the collagen I-rich interstitial spaces, with few in the airways. These observations suggest that CD4 T cell interaction with the tissue microenvironment is distinct from CD8 T cells and support the concept that CD4+ T cells in peripheral tissues are regulated differently than the CD8 subset.

α1β1 Integrin+ and Regulatory Foxp3+ T Cells Constitute Two Functionally Distinct Human CD4+ T Cell Subsets Oppositely Modulated by TNFα Blockade

The expression of the collagen receptor alpha(1)beta(1) integrin (VLA-1) on CD4(+) T cells is largely restricted to CCR7(-)CD45RO(+) cells that localize to inflamed tissues. Moreover, neutralizing alpha(1) integrin, in vivo, has been shown to compromise cell-mediated immunity. Our current study shows that the expression of VLA-1 on human CD4(+) T cells is restricted to conventional effectors. In contrast, Foxp3(+) T regulatory cells (Tregs) do not express this receptor. Moreover, Foxp3 or VLA-1 expression remained a mutually exclusive event in CD4(+) T cells even upon polyclonal anti-CD3-induced activation. Because TNFalpha blockade ameliorates certain T cell-dependent autoimmune disorders in humans, we investigated, in vitro, whether neutralizing TNFalpha affected the balance between the proinflammatory VLA-1(+) effectors and the counteracting Tregs. We found that anti-CD3 stimulation of freshly isolated PBL from healthy individuals, coupled with continuous TNFalpha blockade, inhibited the typical activation-dependent generation of CD4(+)VLA-1(+) Th1 cells. In contrast, it augmented the outgrowth of VLA-1(neg/dim)CD25(high) and Foxp3(+)CD4(+) T cells. Indeed, repeated anti-CD3 stimulation coupled with TNFalpha blockade generated CD4(+) T cell lines enriched for VLA-1(-)Foxp3(+) Tregs. Importantly, these CD4(+) T cells displayed potent suppressive functions toward autologous CD4(+) PBL, including the suppression of the activation-dependent induction of VLA-1(+) effectors. Thus, we propose a novel mechanism by which anti-TNFalpha therapy may restore self-tolerance, by shifting the balance between VLA-1(+) effectors and Foxp3(+) Tregs, during immune activation, in favor of the latter suppressor cell population.

Expression of the alpha1beta1 integrin, VLA-1, marks a distinct subset of human CD4+ memory T cells

The alpha1beta1 integrin, very late antigen-1 (VLA-1), is a collagen receptor expressed in many CD4+ T cells localizing to inflamed tissues. Here we show that the expression of VLA-1 is a stable marker of a distinct subset of CD4+ memory T cells. Thus, in human peripheral blood lymphocytes (PBLs), approximately 1-4% of the CD4+ T cells express VLA-1, and following T cell receptor activation ex vivo, the percentage of VLA-1+ cells increases within the CD45RO+ population. Importantly, the activated VLA-1+ and VLA-1- cells can be isolated and maintained in culture as phenotypically stable subsets. Functionally, CD4+ memory T cells, operationally defined as the cells that divide rapidly following stimulation with a recall antigen, are highly enriched for VLA-1+ cells. Moreover, depletion of the small fraction of VLA-1+ cells present in CD4+ PBLs prior to stimulation significantly abrogated the proliferative response to recall antigens. Notably, the VLA-1+ cells in fresh CD4+ PBLs are composed of resting CD45RO+/RA-, CCR7-, CD62L+, CD25-, and VLA-4hi cells. Interestingly, this VLA-1+ subset is enriched for Th1-type cells, and Th1-polarizing conditions during T cell activation favor the emergence of VLA-1+ cells. Thus, VLA-1 expression is a stable marker of a unique subset of human memory CD4+ T cells that predominantly differentiates into Th1 cells.

β1integrins are distributed in adhesion structures with fibronectin and caveolin and in coated pits

Integrins are found in adhesion structures, which link the extracelullar matrix to cytoskeletal proteins. Here, we attempt to further define the distribution of β1integrins in the context of their association with matrix proteins and other cell surface molecules relevant to the endocytic process. We find that β1integrins colocalize with fibronectin in fibrillar adhesion structures. A fraction of caveolin is also organized along these adhesion structures. The extracellular matrix protein laminin is not concentrated in these structures. The α4β1integrin exhibits a distinct distribution from other β1integrins after cells have adhered for 1 h to extracellular matrix proteins but is localized in adhesion structures after 24 h of adhesion. There are differences between the fibronectin receptors: α5β1integrins colocalize with adaptor protein-2 in coated pits, while α4β1integrins do not. This parallels our earlier observation that of the two laminin receptors, α1β1and α6β1, only αaβ1integrins colocalize with adaptor protein-2 in coated pits. Calcium chelation or inhibition of mitogen-activated protein kinase kinase, protein kinase C, or src did not affect localization of α1β1and α5β1integrins in coated pits. Likewise, the integrity of coated-pit structures or adhesion structures is not required for integrin and adaptor protein-2 colocalization. This suggests a robust and possibly constitutive interaction between these integrins and coated pits.Key words: adhesion, endocytosis, extracellular matrix, microscopy, confocal, signalling.