Gut-specific homing

Lymphocytes migrate from the blood stream via a distinctive 'rolling interaction' mediated by interactions between cellular surface proteins. This 'rolling interaction' occurs in three stages:

1. Initial binding of a lymphocyte selectin and its ligand. This creates an unstable interaction where the lymphocyte is lightly held to the endothelium wall. The force of the lymph causes the cell to characteristically roll along the vessel. An example of this is the interaction between L-selectin and the PNAD (peripheral node addressin) ligand.
2. Arrest of the 'rolling' cell by activation step. Further interactions between proteins or activation via chemokines cause more stable forces to be formed between the cell and the vessel. The cell now is firmly attached to the vessel wall. Continuing the example the cell would be arrested by the interaction between ICAM and LFA-1 which are strengthened by chemokines.
3. Conformational change. Cellular activation causes a conformational change which results in the cell flattening against the vessel wall. Once flattened, the cell can undergo transendothelial migration.[1]

"Homing receptors" are crucial in forming in the interactions which allow lymphocyte movement.

MadCam-1 receptor

MadCAM-1 expression is continuous in the high endothelial venules of Peyer's patches and in the micro-vessels of the intestinal tract.[5] MadCAM-1 inhibitors prevent T cell migration to the gut.

Structural analysis of the MadCAM-1 protein shows that it is a 41.5 kDa transmembrane protein with a small cytoplasmic tail and a large extracellular region. This extracellular region includes three immunoglobin-like (Ig) domains. The last of which has 33% structural identity to the Ca₂ domain of IgA-1. The first two of the domains are predicted to be the most important in the α4β7 interaction.

The MadCAM-1 protein has structural homology to the vascular surface receptors VCAM1 and ICAM at 28% and 32% respectively.[6]

α4β7 integrin is expressed on activated T cell imprinted for gut-specific homing. It is the counter receptor for the MadCAM-1 protein and the interactions of which cause the initial interaction step in 'rolling' cell model of lymphocyte migration

CCR9 receptors are presented on the T cells and are involved in forming a more stable interaction between the lymphocyte and the vessel wall. The ligand for CCR9 proteins is the CCL25 chemokine (which is otherwise known as TECK) protein. CCL25 is a chemokine which, upon binding to its receptor CCR9 on the lymphocyte, activates a conformational change in surface integrins on that lymphocyte,[7] enabling high-affinity binding to the vessel wall and subsequent transmigration.

The CCL25 chemokine is prevalent in the epithelial tissue of the small intestine (7). It has been shown that CD8+ T cells activated in Peyer's patch lymph tissue display a tropism towards the CCL25 chemokine. The T cells migrate up a gradient towards CCL25.[4]

All GALT (gut-associated lymphoid tissue) DC, with an α chain CD103, are capable of producing gut-homing T cells. From this it has been suggested that dendritic cells are imprinted in some way before reaching the lymphoid organs in order to induce homing ability.[8]

Alcohol dehydrogenases (ADHs) and retinal dehydrogenase (RALDH) enzymes catalyse the oxidation of dietary vitamin A to retinoic acid. It is the presence of this retinoic acid which induces the expression of CCR9 and α4β7.[9] Lower concentrations of retinoic acid, inside the cell, result in a decreased expression of the chemokine receptor CCR9 which in turn limits the ability of the cell to enter the gut mucosa. While the expression of α4β7 is also reduced under lower retinoic acid concentrations, it is predicted that the retinoic acid levels must be depleted to a greater extent to affect α4β7 expression compared to CCR9.[10]

Western blot analysis showed dendritic cells from Peyers' patches produce both RALDH1 and ADH proteins. Additionally these have been found expressed in the epithelial cells of the small intestine. It therefore followed that these cells are capable of converting vitamin A into the major physiological form of RA: all-trans RA. Studies showing the DCs ability to catalyze the change of retinol to retinoic acid confirm this assumption.[10]

Vitamin A oxidation

Research on mice showed that the level of retinoic acid in the cells did have a significant effect on the expression of receptors on the cell surface. Mice models showed that even a 0.1 nM increase in the concentration of retinoic acid in cells increased the expression of α4. Furthermore, β7 expression was increased by raising the concentration of cellular retinoic acid. To ensure that increase in RA concentration was due to the catalytic enzymes ADH and RALDH, inhibitors of these enzymes were used. Citral, an inhibitor of the enzyme RALDH, suppressed the expression of α4β7 adhesins on the cell surface suggesting that the vitamin A → retinoic acid catalysis was important.

This, in theory, should increase the homing ability of the activated T cells to the gut. Evidence of this came from studies using RA-treated and vitamin A-deficient mice. In the mice with increased levels of RA there was a higher number of T cells found in the intestinal tissue. Research on vitamin A-deficient mice confirmed the reverse was true as there was significantly lower number of T cells found in the intestinal lamina propria compared to control mice. Contrastingly, there was no such decrease in T cell numbers in the lungs and liver of the deficient mice implying that the RA imprinting pathway is unique to gut-specific homing T cells.[11]

It has theorized that the RA levels affect the expression of the MadCAM-1 and α4β7 receptors by transcriptional regulation.

The RAR and RXR receptors, which are the nucleic receptors for RA, can form heterodimers given specific cellular conditions. In the dimer conformation, the receptors are able to bind RARE or RXRE sequences found in DNA. These genetic regions are often found in the 5' end of genes suggesting they play a regulatory role. In fact they are often found near the beginning or in introns of genes regulated in response to retinoid levels.

Sections of these RARE sequences, and additionally RA boxes, have been found in the first exons of the genes which encode b7 and the CCR9 receptors. Using antagonist proteins against RAR subunit showed a reduction in the expression of alpha.[12]

Therefore, it may be that the expression of the alpha receptors are transcriptionally regulated by the concentration of RA in the cell.

Increased vitamin A concentrations was also shown to reduce the expression of the receptors P-Lig, E-Lig and Fuct-VII in vitro. This suggests that not only does vitamin A increase the ability of the T-cell to home to the gut it also simultaneously reduces the likelihood of the cell to bind to another tissue.