Epithelial IL-15 Is a Critical Regulator of γδ Intraepithelial Lymphocyte Motility within the Intestinal Mucosa

Intraepithelial lymphocytes (IELs) expressing the γδ TCR (γδ IELs) provide continuous surveillance of the intestinal epithelium. However, the mechanisms regulating the basal motility of these cells within the epithelial compartment have not been well defined. We investigated whether IL-15 contributes to γδ IEL localization and migratory behavior in addition to its role in IEL differentiation and survival. Using advanced live cell imaging techniques in mice, we find that compartmentalized overexpression of IL-15 in the lamina propria shifts the distribution of γδ T cells from the epithelial compartment to the lamina propria. This mislocalization could be rescued by epithelial IL-15 overexpression, indicating that epithelial IL-15 is essential for γδ IEL migration into the epithelium. Furthermore, in vitro analyses demonstrated that exogenous IL-15 stimulates γδ IEL migration into cultured epithelial monolayers, and inhibition of IL-2Rβ significantly attenuates the basal motility of these cells. Intravital microscopy showed that impaired IL-2Rβ signaling induced γδ IEL idling within the lateral intercellular space, which resulted in increased early pathogen invasion. Similarly, the redistribution of γδ T cells to the lamina propria due to local IL-15 overproduction also enhanced bacterial translocation. These findings thus reveal a novel role for IL-15 in mediating γδ T cell localization within the intestinal mucosa and regulating γδ IEL motility and patrolling behavior as a critical component of host defense.

Intraepithelial lymphocytes (IELs) are a subset of tissueresident immune cells found above the basement membrane within the intestinal epithelium. Within the small intestine, approximately half of intestinal IELs bear the unconventional gd TCR. Distinct from conventional T cells, gd IELs are considered to bridge innate and adaptive immunity due to their rapid response to commensal and pathogenic microbes (1–3). Based on their close proximity to the intestinal lumen, gd IELs are well positioned to provide the first line of immune surveillance to maintain an intact epithelial barrier.

Studies in gd T cell–deficient mice have demonstrated a largely protective role for gd IELs, as these mice exhibit increased susceptibility to enteric infection and experimental colitis (4–9). Consistent with these findings, gd IELs have been shown to produce antibacterial and antiviral factors (5, 6, 10, 11), as well as growth factors to promote epithelial restitution following injury (8). Although there is only one IEL for every 5–10 epithelial cells, we and others have shown that gd IELs extensively interact with the villous epithelium by actively patrolling the basement membrane and migrating in between adjacent epithelial cells in what we refer to as the lateral intercellular space (LIS) (7, 12–14). The dynamics of gd IEL migration are regulated in part through homotypic interaction of the transmembrane tight junction protein occludin, which is expressed both on gd IELs and epithelial cells, as well as through interactions between epithelial E-cadherin and CD103 (aEb7 integrin) (12). Furthermore, we have shown that this patrolling behavior and migration into the LIS is required for gd IEL–mediated protection against acute invasion of enteric pathogens (7). Impaired gd IEL migration increased early pathogen translocation to a similar extent as was observed in gd T cell– deficient mice, which resulted in a more rapid onset of salmonellosis (7). These findings demonstrated that gd IEL surveillance of the epithelium is critical to the innate immune function of these first responders; however, the molecular mechanisms regulating basal gd IEL migration within the epithelial compartment have yet to be identified.

IL-15 belongs to the four a helix bundle family of cytokines that includes IL-2, IL-4, IL-7, IL-9, and IL-21 (15). Although IL-15 shares the common g-chain receptor (CD132) with all of these family members, only the b-chain receptor (IL-2Rb, CD122) is shared with IL-2 (16). IL-15 is unique in that following synthesis in the endoplasmic reticulum, IL-15 and IL-15Ra form a complex that is trafficked through the Golgi to the plasma membrane (17), where the complex is transpresented directly to IL-2Rb/gC on the opposing cell. Within the intestine, IL-15/IL-15Ra complexes are most highly expressed in epithelial cells and lamina propria dendritic cells (18); however, the transpresentation of IL-15 by epithelial IL-15Ra to IL-2Rb expressed on T cells is required for the proliferation and survival of CD8aa+ TCRab+ and CD8aa+ TCRgd+ IEL populations (19–21). Elevated IL-15 expression has been observed in patients with inflammatory bowel disease (IBD) (22, 23) or celiac disease (24–26), and, unsurprisingly, given the role of IL-15 in T cell proliferation, IEL number is increased in both diseases (27, 28).