Pinkie, the First Viable Germline Hypomorph Allele of Retinoid X Receptor Alpha, Reveals an Important Role for RXRa in Th2 Development.

Pinkie is a recessive ENU-induced mutation, homozygotes develop premature graying and subsequently, progressive alopecia. Histologically, the skin is characterized by the destruction of hair follicle architecture, formation of dermal cysts and appearance of black papules throughout the ventral skin late in life. The Pinkie mutation was mapped to the retinoid X receptor alpha (Rxra) locus (chromosome 2) on 700 meioses and positionally cloned. As a component of the nuclear receptors for retinoic acid and other bioactive molecular ligands, RXRa plays an irreplaceable role during embryogenesis and a null allele of RXRa invariably causes embryonic lethality. Pinkie, which results from the amino acid substitution N273I in the ligand-binding domain, is the first viable germline hypomorphic mutation of RXRa. Transfection studies suggest that the abnormal protein retains only 10 to 20% of the activity of the wild type receptor. Pinkie homozygotes develop a progressive phenotype, part of which entails a severe, age-dependent imbalance of Th1/Th2 differentiation upon antigenic challenge. When immunized with ovalbumin (OVA), pinkie homozygotes displayed markedly decreased IgG1 levels in serum as compared with normal C57BL/6 mice; in vitro OVA re-stimulation of pinkie splenocytes causes greatly enhanced IFN-g production and decreased IL-4 production. Upon MCMV infection, pinkie homozygotes showed significantly higher type I interferon levels in serum than normal C57BL/6 mice. The in vitro differentiation of naïve CD4 T cells demonstrated skewed Th1 development in pinkie mouse compared to that of C57BL/6 control animal. However, real-time PCR didn’t reveal significant difference in mRNA level of GATA-3 and T-bet, which are believed to be critical transcription factors in Th1/Th2 development. These data collectively imply that RXRa is required for Th2 development. This requirement might reflect an influence of RXRa on the expression of pertinent transcription factors (e.g., GATA-3, T-bet), cytokines (IL-4, IFN-gamma) and/or cytokine receptors that are involved in Th1/Th2 differentiation. RXRa represents a likely interface between the nutritional environment and the immune status of the host. Consumption of retinoids and vitamin D may therefore be expected to influence the development of the allergic/atopic (Th2) phenotype, and provide a plausible explanation for the increasing frequency of atopic disease observed in developed countries.