Retinoids Selectively Induce Gut-Homing Properties in Cutaneous T Cell Lymphoma (CTCL) Cells

Introduction: Cutaneous T cell lymphoma (CTCL) is a heterogeneous group of malignancies characterized by accumulation of malignant T cells within skin. Retinoids, metabolic derivatives and synthetic analogs of vitamin A, naturally imprint gut homing in T lymphocytes by inducing integrin alpha4beta7 and CCR9 expression which effectively targets the immune cells away from the skin niche. Although successfully used in CTCL therapy for decades, the therapeutic mechanism of action of retinoids in resolving CTCL remains largely speculative. How the natural role of retinoids in immune cell trafficking relates or applies to therapeutic effectiveness in CTCL has not been addressed and merits investigation. Here we provide the first evidence that the natural role of retinioids in governing immune cell localization to the gut accounts for their therapeutic benefit in CTCL.

Methods: Changes in cell surface integrin levels of T cell lineages, 2 CTCL cells lines (MJ and HuT78) and 3 non-CTCL cell lines (Jurkat, Molt4 and CCRFCEM), exposed to retinoid was determined by flow cytometry. Static cell adhesion assays to various immune trafficking ligands were performed to establish if retinoids bias integrin function that would occlude skin localization and occupancy. Function blocking or activating monoclonal antibodies against specific integrin subunits were employed to delineate the specific integrin heterodimers required for retinoid-prompted adhesion. Static adhesion results were verified in soluble binding assays. TransWell migration assays were utilized to address changes in cell migration that occurred upon retinoid exposure.

Results: Retinoids induced integrin beta7 subunit expression on CTCL cell surfaces and enhanced beta7-mediated cell adhesion in multiple CTCL cell lines, but not in non-CTCL lineages. The retinoid-induced adhesion observed was selective as adhesion increased to the gut-specific ligand MAdCAM-1, but not to other trafficking ligands such as VCAM-1 or fibronectin. Interestingly, retinoid-induced CTCL cell adhesion to MAdCAM-1 was potently attenuated by 1,25-dihydroxyvitamin D₃, a metabolic vitamin derivative involved in inducing skin-homing immune properties. Of clinical relevance, this work entailed the use of Bexarotene, an RXR agonist and the only FDA-approved retinoid for CTCL therapy. Since immune cell trafficking involves the progressive involvement of a variety of protein ligands and receptors including chemokine receptors, we extended our study to address the migratory properties of CTCL cells. Bexarotene significantly increased the migration of CTCL cells towards the gut chemokine CCL25/TECK as compared to vehicle treated cells.

Conclusions: Retinoids imprint gut homing properties selectively within CTCL cells by inducing beta 7 integrin expression and function, which may account, in part, for the therapeutic benefit of retinoids in CTCL. Retinoids, through their natural role in establishing and maintaining mucosal immunity, plausibly occlude further trafficking or terminate the residency of the malignant T cells within the skin niche. This action may predispose and heighten the susceptibility of malignant cells to additional therapies (e.g. UV light) commonly used in combination with Bexarotene. Our data establish that the natural role of retinoids in immune cell localization embody an unexplored therapeutic mechanism in CTCL treatment.