Radiation and host retinoic acid signaling promote the induction of gut-homing donor T cells after allogeneic hematopoietic stem cell transplantation.

Intestinal graft-versus-host disease (GVHD) remains a devastating complication after allogeneic hematopoietic stem cell transplantation (HSCT). Although it has been well established that gut-tropic donor T cells expressing integrin α4ß7 are required to cause intestinal damage, the factors that control the induction of this pathogenic T cell population remain to be identified. Retinoic acid (RA) plays an important role in inducing α4ß7 expression on T cells. In this study, we showed that gene expression of retinaldehyde dehydrogenase, the key enzyme involved in RA biosynthesis, is significantly increased in the spleen and mesenteric lymph nodes (MLNs) of irradiated mice. In a C57BL/6-into-B6D2F1 allogeneic HSCT model, irradiation significantly increased the induction of α4ß7+ -donor T cells in mesenteric lymph nodes and spleen. Furthermore, we found that the RA pathway modulates the ability of dendritic cells to imprint gut-homing specificity on alloreactive T cells. We also showed that host dendritic cell RA signaling influences GVHD risk. Our studies identified radiation and recipient RA signaling as 2 primary factors that dictate the magnitude of gut-homing donor T cell induction after allogeneic HSCT. Attenuating radiation-associated inflammation and modulating host RA signaling represent feasible strategies to mitigate intestinal GVHD by reducing gut-seeking pathogenic donor T cells.

Effects of Donor Vitamin A Deficiency and Pharmacologic Modulation of Donor T Cell Retinoic Acid Pathway on the Severity of Experimental Graft-versus-Host Disease.

Graft-versus-host disease (GVHD) is the major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). A combination of genetic and nongenetic factors dictates the incidence and severity of GVHD. Recent studies have identified the potential role of the retinoic acid (RA)/retinoic acid receptor (RAR) pathway in the pathogenesis of GVHD. RA is the active metabolite of vitamin A. Thus, a clinically relevant question is whether HSCT donor and/or recipient vitamin A status affects the development of GVHD. It has been previously reported that recipient vitamin A deficiency is associated with reduced intestinal GVHD and prolonged overall survival after experimental allogeneic HSCT. However, it is still unknown whether donor vitamin A status influences GVHD development. In the current study, we report that chronic vitamin A deficiency changes the composition of T cell compartment of donor mice with a reduction in the percentage of CD4+ T cells. We showed that although vitamin A deficiency does not affect donor T cell alloreactivity on a per cell basis, a decreased proportion of donor CD4+ T cells in marrow graft inoculums leads to reduced incidence and severity of GVHD. Furthermore, our proof of principle studies using a pan-RAR antagonist demonstrated that transient inhibition of donor T cell RAR signaling can reduce T cell alloreactivity and their ability to cause lethal GVHD. Our studies provide preclinical evidence that donor vitamin A deficiency may be a nongenetic factor that can modulate the severity of GVHD and pharmacologic interfering RA/RAR pathway in donor T cells might be a valuable approach for mitigating GVHD after allogeneic HSCT.

A critical role for the retinoic acid signaling pathway in the pathophysiology of gastrointestinal graft-versus-host disease.

Damage to the gastrointestinal tract during graft-versus-host disease (GVHD) is one of the major causes of morbidity and mortality in allogeneic hematopoietic stem cell transplant (HSCT) recipients. In the current study, we identified a critical role for the retinoic acid (RA) signaling pathway in the induction and propagation of gastrointestinal GVHD. The administration of exogenous RA significantly increased expression of the gut-homing molecules, CCR9 and α4ß7, on donor T cells in mesenteric lymph nodes, and augmented the accumulation of proinflammatory CD4(+) and CD8(+) T cells within the gut mucosa, leading to a selective exacerbation of colonic GVHD and increased overall mortality. Conversely, depletion of RA in recipient mice by vitamin A deprivation resulted in a dramatic reduction of gut-homing molecule expression on donor T cells after HSCT. Significantly, absence of the RA receptor-α on donor T cells markedly attenuated the ability of these cells to cause lethal GVHD. This observation was attributable to a significant reduction in pathological damage within the colon. These findings identify an organ-specific role for RA in GVHD and provide evidence that blockade of the RA signaling pathway may represent a novel strategy for mitigating the severity of colonic GVHD.

Circadian genes, xBmal1 and xNocturnin, modulate the timing and differentiation of somites in Xenopus laevis.

We have been investigating whether xBmal1 and xNocturnin play a role in somitogenesis, a cyclic developmental process with an ultradian period. Previous work from our lab shows that circadian genes (xPeriod1, xPeriod2, xBmal1, and xNocturnin) are expressed in developing somites. Somites eventually form the vertebrae, muscles of the back, and dermis. In Xenopus, a pair of somites is formed about every 50 minutes from anterior to posterior. We were intrigued by the co-localization of circadian genes in an embryonic tissue known to be regulated by an ultradian clock. Cyclic expression of genes involved in Notch signaling has been implicated in the somite clock. Disruption of Notch signaling in humans has been linked to skeletal defects in the vertebral column. We found that both depletion (morpholino) and overexpression (mRNA) of xBMAL1 protein (bHLH transcription factor) or xNOCTURNIN protein (deadenylase) on one side of the developing embryo led to a significant decrease in somite number with respect to the untreated side (p<0.001). These manipulations also significantly affect expression of a somite clock component (xESR9; p<0.05). We observed opposing effects on somite size. Depletion of xBMAL1 or xNOCTURNIN caused a statistically significant decrease in somite area (quantified using NIH ImageJ; p<0.002), while overexpression of these proteins caused a significant dose dependent increase in somite area (p<0.02; p<0.001, respectively). We speculate that circadian genes may play two separate roles during somitogenesis. Depletion and overexpression of xBMAL1 and NOCTURNIN both decrease somite number and influence expression of a somite clock component, suggesting that these proteins may modulate the timing of the somite clock in the undifferentiated presomitic mesoderm. The dosage dependent effects on somite area suggest that xBMAL1 and xNOCTURNIN may also act during somite differentiation to promote myogenesis.