**survival of IEL, we performed a similar experiment but the potential source of osteopontin were TCR+ or TCR+ IEL (Fig. responses such as survival of Th17 cells and homeostasis of NK cells, among others. Because of its impact in the immune system, we investigated the role of osteopontin in the homeostasis of IEL. Here, we report that mice deficient in the expression of osteopontin exhibit reduced numbers of the IEL subpopulations TCR+, TCR+CD4+, TCR+CD4+CD8+ and TCR+CD8+ cells AZD5153 6-Hydroxy-2-naphthoic acid in comparison to wild-type mice. For some IEL subpopulations the decrease in cells numbers could be attributed to apoptosis and reduced cell division. Moreover, we show that exogenous osteopontin stimulates the survival of murine IEL subpopulations and unfractionated IEL derived from human intestines, an effect mediated by CD44, a known osteopontin receptor. We also show that iCD8 IEL, but not TCR+ IEL, TCR+ IEL or intestinal epithelial cells, can promote survival of different IEL populations via AZD5153 6-Hydroxy-2-naphthoic acid osteopontin, indicating an important role for iCD8 cells in the homeostasis of IEL. Introduction One of the largest immunological compartments in the body is usually comprised of intraepithelial lymphocytes (IEL), a group of immune cells interspaced between the monolayer of intestinal epithelial cells (IEC). IEL can be divided into two groups based on T cell receptor (TCR) expression (1C3). TCR+ IEL express or chains. TCR+ IEL can be further subdivided into TCR+CD4+, TCR+CD4+CD8+, TCR+CD8+, and TCR+CD8+ cells. TCRneg IEL comprise innate lymphoid cells (ILC) (4C6) and lymphocytes characterized by expression of intracellular CD3 chains (iCD3+), some of which express CD8 (iCD8 cells) (7, 8). Because of their anatomical location, IEL function as sentinels between the antigenic contents of the intestinal lumen and the sterile environment under the basal membrane of the epithelium. Indeed, TCR IEL surveil for pathogens (9), secrete antimicrobials conferring protection against pathobionts (10), and protect from intestinal inflammation (11). Other IEL, like conventional CD8 T cells that migrate into the epithelium, can protect against contamination (12) and reside in this organ as memory cells (13, 14). TCR+CD4+CD8+ IEL can prevent development of disease in the T cell adoptive transfer model of colitis (15). iCD8 cells AZD5153 6-Hydroxy-2-naphthoic acid confer protection against infection and may protect AZD5153 6-Hydroxy-2-naphthoic acid against necrotizing enterocolitis in neonates (8), but these cells can also promote intestinal inflammation in some experimental conditions (16). iCD3+ IEL are involved in malignances associated with celiac disease (7). Osteopontin is usually a glycosylated phosphoprotein encoded by the Spp-1 (secreted phosphoprotein) gene, originally characterized as part of the rat bone matrix (17, 18). Osteopontin is usually a versatile molecule involved in many physiological and disease processes (19C21). The role of osteopontin in intestinal inflammation is usually diverse. For example, Spp-1-deficient mice present with milder disease in the trinitrobenzene sulphonic acid and DSS models of colitis (22, 23). In humans with inflammatory bowel diseases (IBD), plasma osteopontin is usually significantly increased AZD5153 6-Hydroxy-2-naphthoic acid compared to healthy individuals (24, 25). Some reports indicate that osteopontin is usually downregulated in the mucosa of Crohns disease (CD) patients (26), whereas other groups have reported higher osteopontin expression in the intestines of individuals with CD and ulcerative colitis (UC) compared with healthy controls (25, 27). Because of its involvement in IBD, this molecule could be a potential biomarker (28) and has been explored as a therapeutic target in clinical trials (29). These reports clearly underscore the importance of osteopontin in intestinal inflammation and warrant further investigation of this molecule in mucosal immune responses. Studies of osteopontin in the immune system have provided important insight into the role of this molecule. For example, osteopontin is usually involved in macrophage chemotaxis (30), inhibition of NK cell apoptosis and promotion of NK cell responses (31), as well as modulation of dendritic cell function (32). In terms of T cells, osteopontin has been shown to stimulate the survival of concanavalin A-activated lymph node T cells neutralization of IEL-derived osteopontin resulted in decreased survival of TCR and TCR IEL (35), confounding the results. Our group has recently shown that iCD8 IEL enhance the survival Narg1 of ILC1-like IEL, via osteopontin, impacting the development of intestinal inflammation (36). Here, we hypothesize that osteopontin and iCD8 cells are key components involved in the homeostasis of most IEL populations. In the present report, we investigated this hypothesis by carefully studying the role of osteopontin in the homeostasis of different IEL subpopulations in mice and total IEL derived from human tissue. We present data showing that.