Regulation of Antigen-Experienced T Cells: Lessons from the Quintessential Memory Marker CD44

Abstract

Despite the widespread use of the cell-surface receptor CD44 as a marker for antigen (Ag)-experienced, effector and memory T cells, surprisingly little is known regarding its function on these cells. The best-established function of CD44 is the regulation of cell adhesion and migration. As such, the interactions of CD44, primarily with its major ligand, the extracellular matrix (ECM) component hyaluronic acid (HA), can be crucial for the recruitment and function of effector and memory T cells into/within inflamed tissues. However, little is known about the signaling events following engagement of CD44 on T cells and how cooperative interactions of CD44 with other surface receptors affect T cell responses. Recent evidence suggests that the CD44 signaling pathway(s) may be shared with those of other adhesion receptors, and that these provide contextual signals at different anatomical sites to ensure the correct T cell effector responses. Furthermore, CD44 ligation may augment T cell activation after Ag encounter and promote T cell survival, as well as contribute to regulation of the contraction phase of an immune response and the maintenance of tolerance. Once the memory phase is established, CD44 may have a role in ensuring the functional fitness of memory T cells. Thus, the summation of potential signals after CD44 ligation on T cells highlights that migration and adhesion to the ECM can critically impact the development and homeostasis of memory T cells, and may differentially affect subsets of T cells. These aspects of CD44 biology on T cells and how they might be modulated for translational purposes are discussed.

Keywords: CD44; T cell; extracellular matrix; memory; migration.

Figure 1

CD44-mediated rolling and arrest. CD44-dependent adhesion mechanisms are important for the mobilization of effector T cells in sites of infection and inflammation. At these sites, endothelial cells secrete chemokines and express selectins (E- and P-selectin), HA, and integrin ligands (e.g., VCAM-1) in response to inflammatory cytokines (left panel). CD44 can mediate rolling interactions with vascular endothelial cells that express HA, its natural ligand, or even E-selectin (center panel). Chemokine signaling via GPCRs results in increased integrin affinity, such as VLA-4, which enhances adhesiveness [“inside-out” signaling (a)]. Engagement of CD44 with HA may also trigger GPCR-dependent “inside-out” upregulation of VLA-4–VCAM binding (b). Furthermore, CD44 may associate with VLA-4 in the membrane of activated T cells and augment signaling [(c) right panel]. Thus, CD44 contributes to mechanisms that allow T cells to successfully interpret environmental signals to migrate into sites of inflammation.

Figure 2

CD44 is critically involved during firm adhesion and intravascular crawling to mediate transmigration of T cells. CD44 engagement activates the non-receptor tyrosine kinase, Pyk2, in a PI3K dependent fashion and mediates cell spreading during firm adhesion (left panel). In addition to Pyk2, CD44 engagement activates the Src kinase, Lck, which is required for actin polymerization (center panel). Further, cytoskeletal rearrangement is mediated by the ERM proteins, which are also induced following PI3K signaling. CD44 can directly associate with the ERM proteins, which crosslink CD44 and other proteins to the actin cytoskeleton. The development of cell polarity and the diapedesis occurs through PECAM and JAM that enable extrusion of filopodia into the junctions of endothelial cells and transmigration into tissue (center and right panel). Thus, signaling via CD44 can promote the cytoskeletal changes necessary for T cell extravasation from the blood into tissues.

Figure 3

T cell motility and activation in situ is modulated by CD44. Following diapedesis, T cells adopt an amoeboid shape and migrate in a polarized fashion along collagen fibrils in response to a chemotactic gradient toward the site of infection/inflammation (left panel). Whereas chemotactic receptors are located in the leading edge, CD44 localizes in the uropod, which is in contact with the ECM, to maintain a polarized shape. As movement takes place, CD44 anchorage of the cell’s uropod to the ECM is disengaged. Migration through the parenchymal basement membrane is facilitated by enzymatic digestions mediated by secretion of MMPs, which may even be bound to CD44 for directed proteolytic activity. Following efficient migration to areas of infection, T cells will encounter, recognize, and respond to antigen (right panel). CD44 augments TCR signaling by increasing the availability of Lck for downstream engagement of ZAP-70. CD44 ligation activates the transcription factor, NF-κB, and induces expression of IFN-γ highlighting its contribution to T cell activation.

Figure 4

CD44 potentiates T cell activation, expansion, contraction, and maintenance of memory cells. Besides playing a pivotal role during recruitment, CD44 ligation affects T cell memory function by augmenting activation (A), ensuring survival (B), and regulating homeostasis (C) to facilitate maximum expansion following challenge (D). Following recognition of Ag via TCR, CD44 affinity and expression are upregulated and it localizes with the TCR signaling complex in the immunological synapse where it receives signals from HA presented by CD44 expressed on DCs (panel 1). CD44 ligation contributes to and enhances signal transduction in the context of TCR engagement by increasing the availability of Lck for downstream engagement of ZAP-70, which transmits the signals necessary for T cell activation. Signaling events in the primary response to infections can profoundly affect the generation and maintenance of T cell memory (panel 2). HA binding to CD44 can induce a survival signal via the PI3K/Akt pathway, which may inhibit apoptosis during contraction by interfering with cell death either through direct obstruction of Fas to prevent DISC assembly and caspase 8 activation or by affecting Fas signaling itself. Although differences in the capacity of LMW versus HMW HA to elicit CD44-mediated signaling in T cells has not been carefully investigated, recent studies suggest that HMW (red bar), but not LMW (green bar), HA (A–D) can upregulate the transcription factor FoxP3 in T_regs cells via CD44 signaling (panel 3). CD44-regulated FoxP3 expression was linked to a greater capacity for IL-10 production and the development of surface TGF-β1 expression, which are important for their regulatory function. We hypothesize that effector T cells are more likely to respond to LMW HA generated during inflammation. Furthermore, during homeostasis, we envision that HA binding by CD44 elicits enhanced survival due to engagement of Lck (panel 4). CD44 signaling may increase the responsiveness of IL-7Rα by making more Lck available leading to IL-7-mediated upregulation of Bcl-2 via Stat5 signaling. Thus, CD44 could have a global function in regulating the transition of effector cells to memory cells, as well as in maintaining memory T cells that exceeds its roles in migration and motility.