Cyp11a1 or P450scc is a mitochondrial side-chain cleavage enzyme required for the conversion of cholesterol to pregnenolone. factors T-box expressed in T cells (T-bet) hJumpy or GATA binding protein 3 (GATA3). Adoptive transfer of aminoglutethimide-treated CD8+ T cells into sensitized and challenged CD8-deficient recipients failed to restore airway hyperresponsiveness and inflammation. We demonstrate that Cyp11a1 controls the phenotypic conversion of CD8+ T cells from IFN- to IL-13 AS-604850 production, linking steroidogenesis in CD8+ T cells, a nonclassical steroidogenic tissue, to a proallergic differentiation pathway. Asthma has increased dramatically over the past 50 y and now affects 5C10% of the population in many developed countries (1). National and international guidelines recommend the use of inhaled corticosteroids as the first AS-604850 step in controlling airway inflammation and symptoms in persistent asthma (2, 3). However, it has been demonstrated that 45% of steroid-naive asthmatic patients do not respond to inhaled corticosteroids. Corticosteroid insensitivity has been adopted as a principal criterion for characterizing asthma severity (4). Increased numbers of CD8+ T cells, which are more resistant than CD4+ T cells to corticosteroids (5), have been detected in steroid-insensitive asthmatics (6) AS-604850 and have correlated with AS-604850 lower lung function (7). We and others also found that numbers of CD8+IL-13+ cells were increased in experimental asthma models in mice (8C10) as a result of their activation by IL-4Cproducing CD4+ T cells (11). CD8+ T cells can be polarized to effector subsets with cytokine profiles similar to those found in CD4+ T cells (12C14). Both in vivo and in vitro, IL-4 was capable of triggering CD8+ T-cell differentiation from a predominant IFN-Cproducing cell to one producing IL-13. This conversion was associated with suppression of T-box expressed in T cells (T-bet) and induction of GATA binding protein 3 expression and was characterized by enhanced activating histone modifications and RNA polymerase (Pol) II recruitment to the GATA3 and IL-13 loci. IL-13 transcription only occurred at a later stage following T-cell receptor (TCR) stimulation, indicating that IL-4Cinduced GATA3 recruitment poised the IL-13 locus for TCR-mediated transcription (15). Transcriptional profiling identified cholesterol side-chain cleavage P450 enzyme (Cyp11a1) transcripts as one of the most highly up-regulated during the differentiation of CD8+ T lymphocytes to a T-cell type 2 (Tc2) phenotype, that is, a CD8+ T-cell capable of IL-13 production. Cyp11a1 or P450scc is a mitochondrial side-chain cleavage enzyme required for the conversion of cholesterol to pregnenolone. It catalyzes the initial and rate-limiting step in the synthesis of AS-604850 steroid hormones in tissues with steroidogenic potential (16, 17). Induction of the Cyp11a1 promoter by epidermal growth factor involves a ras/MEK1/AP-1Cdependent pathway (18). Mutations in the Cyp11a1 gene result in steroid hormone deficiency and can cause the rare and a potentially fatal condition, lipoid congenital adrenal hyperplasia (19, 20). In the present study, the role of Cyp11a1 in controlling IL-4Cmediated CD8+ T-cell conversion in vitro and in vivo was examined. It was demonstrated that mRNA transcript levels, protein levels, and the enzyme activity of Cyp11a1 in CD8+ T cells were all increased dramatically following differentiation in the presence of IL-2 plus IL-4 (IL-2+IL-4) compared with IL-2 alone. Furthermore, the Cyp11a1 enzyme inhibitor aminoglutethimide (AMG) or knock down of Cyp11a1 protein levels using a specific shRNA blocked the functional conversion of CD8+ T cells from IFN-C to IL-13Cproducing cells. Expression of the lineage-specific transcription factors T-bet or GATA3 was not affected by inhibition of Cyp11a1 activity, indicating that it was downstream of expression of these master regulatory transcription factors. Adoptive transfer of AMG-treated CD8+ T cells, in contrast to untreated CD8+ T cells, failed to restore airway hyperresponsiveness (AHR) and inflammation in.