F-actin staining of chemokine-stimulated CD8⁺ T cells from (A) wild-type and (B) LPL^−/− mice.

Image courtesy of Dr. Celeste Morley and Dr. Paul Allen, Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri, USA.

Chemokine signalling stimulates T cell motility by mediating cytoskeletal rearrangements and subsequent cell polarity. Although many molecules have been identified in chemokine signalling and regulation of actin polymerization, the molecular mechanisms linking these processes with movement of T cells is unclear. Paul Allen, Eric Brown and colleagues now report in the Journal of Immunology that the actin-bundling protein L-plastin (LPL) mediates chemokine receptor 7 (CCR7)-induced motility by maintaining T cell polarity independently of proximal CCR7 signalling.

Using transgenic models of LPL knock-out (LPL^−/−) mice, the authors first investigated whether LPL was required for thymocyte development and T cell motility. Flow cytometry analyses revealed that LPL deficiency resulted in phenotypically more mature thymocytes and fewer mature CD4⁺ T cells; intrathymic FITC injections confirmed these findings, suggesting a diminished thymic egress in the absence of LPL. Interestingly, in vitro chemotaxis transwell assays showed that T cell migration was reduced by LPL deficiency, in common with earlier reports of reduced motility in the absence of CCR7. Consistently, assays with nontransgenic mice also demonstrated that LPL deficiency results in loss of normal migration and diminished lymph node development in vivo. These data indicate an important role for LPL during thymocyte development and motility.

CCR7 promotes T cell motility by activating small GTPases, such as Rac, which induces actin polymerization. To test whether LPL is involved in CCR7 signalling, the authors investigated the levels of activated Rac and F-actin polymerization in LPL-deficient T cells. Furthermore, they measured the effect of LPL deficiency in CCR7-mediated cell–cell interactions and T cell receptor activation. Surprisingly, no change was detected in either of these processes, which indicated that LPL is dispensable during CCR7-mediated signalling, adhesion and actin polymerization.

So what is the role of LPL in CCR7-mediated T cell motility? Cell polarization is a prerequisite for migration; chemokine receptors, integrins and F-actin all need to be stabilized at the leading edge to form functional lamellipodia and to promote efficient cell motility. Consistently, confocal imaging showed that upon chemokine stimulation, colocalization of F-actin and CCR7 in the lamellipod was reduced in LPL^−/− cells. Furthermore, localized staining of the uropod marker CD43 was also reduced in LPL-deficient cells. This indicates that LPL maintains T cell motility by specifically regulating the stability and polarization of F-actin and CCR7 at the lamellipodia, independently of Rac activation and F-actin polymerization.

This study demonstrates a new regulatory mechanism for T cell motility beyond CCR7 signalling and actin polymerization, where LPL is a critical molecule providing maximal cell polarization in response to chemical cues. It would be interesting to see whether the actin bundling activity of LPL is required in this role.

Author: Iley Ozerlat - Copyright © 2010 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission