The Tetraspanin CD53 Promotes CXCR4 Signaling to Facilitate B Cell Migration

Tetraspanins are transmembrane proteins important for the molecular organization of cellular membranes, and they influence a myriad of processes such as adhesion, migration and activation through interacting with and arranging their partner proteins into signal transducing microdomains. CD53 is one of the important members of this superfamily expressed almost exclusively on hematopoietic cells, with highest expression on mature B cells. We and others have previously shown that CD53 plays a crucial role in B cell development vis promotion of IL7R signaling (Greenberg et al, 2020), and also in promoting lymphocyte recirculation by stabilizing L-selectin (Demaria et al, 2020). Here, we describe a new role for CD53 in the regulation of both normal and malignant B cell trafficking via promotion of CXCL12/CXCR4 signaling.

To assess the role of CD53 in B cell trafficking, we first isolated splenic B cells from WT and Cd53^-/- mice and investigated their ability for both in vitro and in vivo migration and homing. We found that Cd53^-/- B cells had significantly 35% reduced migration toward the chemokine CXCL12 in a transwell assay. Consistent with this finding, Cd53^-/- B cells also had a 60.6% reduction in their ability to transmigrate on human dermal microvascular endothelial cells compared to WT B cells, and they displayed approximately 60% reduced homing to the bone marrow and spleen following transfer into WT recipient mice. To assess whether malignant B cells are similarly dependent on CD53 for proper migration and homing, we crossed the Cd53^-/- mice to the Em-myc transgenic mouse leukemia/lymphoma model mice which express the Myc oncogene under the control of the B-cell specific IgH promoter. While loss of CD53 did not affect survival of the Em-myc mice, we noted a striking difference in the tropism of the malignant cells. Specifically, while Em-Myc;Cd53^-/- mice develop lymphomas, they rarely have marrow involvement, suggesting that CD53 is important for malignant B cell marrow homing as well. Furthermore, in a competitive model where Em-Myc and Em-Myc;Cd53^-/- cells were transplanted together into WT recipients, we found that the tumors, when originating from the Em-myc;Cd53^-/- donor population, nearly always spared the bone marrow, but when they originated from the Em-myccells with normal CD53 expression, they almost always involved the marrow.

We next assessed whether the loss of CXCL12-directed migration in the absence of CD53 is due to reduced receptor expression or function. While surface levels of the CXCL12 receptor, CXCR4, were actually modestly increased on Cd53^-/- B cells compared to controls, signaling through the receptor was significantly impaired. Specifically, CXCL12 normally induces the phosphorylation of Akt and Erk, as well as the rapid internalization of the receptor. These processes were impaired in response to ex vivo CXCL12 stimulation in the absence of CD53, with reduced Akt and Erk phosphorylation as measured by flow cytometry and Western blot, and delayed receptor internalization. Finally, we detected a physical interaction between CD53 and CXCR4 using the Duolink proximity ligation assay, identifying CXCR4 as a novel binding partner of CD53.

Together, these data support a model whereby CD53 interacts with CXCR4 to promote signaling in B cells. Both normal and malignant B cells rely on CXCL12/CXCR4 signaling for proper trafficking and function, and thus CD53 may represent a novel target for the inhibition of malignant B cells.

No relevant conflicts of interest to declare.