Inflammatory bowel disease (IBD) is an inflammatory chronic disease in the gastrointestinal tract (GUT). In the United States, there are approximately 1.4 million IBD patients. It is generally accepted that a dysregulated immune response to gut bacteria initiates the disease and disrupts the mucosal epithelial barrier1,2. For this reason, currently available U.S. Food and Drug Administration (FDA)-approved drugs inhibit the functions of inflammatory mediators or block the homing of immune cells into the gut. However, the inflammatory mediators and immune cells that are targeted are also necessary for immune defenses. As a result, the inflammatory mediator inhibitors compromise systemic immune defense and the immune cell homing blockers weaken gut immune defense, both of which can lead to severe consequences3,4. In addition, the immune cell homing blockers can also block the homing of regulatory T (Treg) cells into the gut and hence can worsen the already compromised gut immune tolerance in IBD patients. Furthermore, blocking of Treg cell homing into the gut may also lead to systemic immune suppression due to the accumulation of Treg cells in the blood5. Finally, inhibitors and blockers function transiently and, thereby, require frequent administrations. Frequent administration of these inhibitors and blockers may further exacerbate the untoward side effects.
Recently, we proposed a novel strategy that can potentially mitigate or even eliminate the side effects associated with current drugs for IBD treatment6. This strategy augments the induction of gut-homing Treg cells in peripheral lymphoid tissues6. The rationale of this strategy is that gut-homing Treg cells specifically home to the gut and hence will not compromise systemic immune defenses. In addition, since Treg cells can potentially form memory7,8, gut-homing Treg cells can potentially provide a stable control of the chronic gut inflammation in IBD patients and, thereby, treatment should not need to be administered as frequently. Furthermore, since this strategy augments the induction of gut-homing Treg cells in vivo, it does not have the concern of in vivo instability in a highly proinflammatory environment that is associated with adoptive transfer of in vitro generated Treg cells9,10. In this regard, in vitro generated Treg cells are one of the proposed strategies for the treatment of autoimmune diseases11,12,13 and transplant rejection14,15. Finally, in this strategy, dendritic cells (DCs) are engineered to produce locally high concentrations of two molecules de novo: active vitamin D (1,25-dihydroxyvitamin D or 1,25[OH]2D) and active vitamin A (retinoic acid or RA). We chose 1,25(OH)2D and RA because 1,25(OH)2D can induce the expression of regulatory molecules (e.g., forkhead box P3 [foxp3] and interleukin-10 [IL-10])16,17 and that RA can stimulate the expression of gut-homing receptors in T cells18. Because both 1,25(OH)2D and RA can also tolerize DCs28,29, we reason that the engineered DCs will be stably maintained in a tolerogenic status in vivo and hence circumvent the in vivo instability concerns that are associated with in vitro generated tolerogenic DCs (TolDCs)19,20,21. In this respect, TolDCs are also one of the proposed strategies for in vivo augmentation of Treg cell functions19,20,21. To support our reasoning, we have shown that the engineered DCs, upon in vivo delivery, can augment the induction of gut-homing Treg cells in peripheral lymphoid tissues6.
An additional advantage of our proposed strategy is that 1,25(OH)2D also has other functions that can potentially benefit IBD patients. These other functions include the ability of 1,25(OH)2D to stimulate the secretion of antimicrobials22 and to suppresses carcinogenesis23. Infections and cancers are frequently associated with IBD24,25.
To generate the DCs that can produce locally high concentrations of both 1,25(OH)2D and RA de novo, we use a lentiviral vector to engineer DCs to overexpress two genes. One gene is the cytochrome P450 family 27 subfamily B member 1 (CYP27B1) that encodes 25-hydroxyvitamin D 1α-hydroxylase (1α-hydroxylase), which physiologically catalyzes the synthesis of 1,25(OH)2D. The other gene is the aldehyde dehydrogenase 1 family member A2 (ALDH1a2) that encodes retinaldehyde dehydrogenase 2 (RALDH2), which physiologically catalyzes the synthesis of RA6.
Because in vivo augmentation of gut-homing Treg cell induction is potentially important in the treatment of IBD, in the following protocol we will detail the procedures for the generation of the 1α-hydroxylase-RALDH2-overexpressing DCs (DC-CYP-ALDH cells) that can be used for the future investigation of gut-homing Treg cells in vivo.