The Atypical Chemokine Receptor CRAM Mediates CCL19 Transcytosis through Endothelial Cells and Modulates CCL19 Activation of Non-Hodgkin Lymphoma B Cells.

Chemokines work as cellular recruitment molecules. Specific combinations of chemokines, receptors, and adhesion molecules determine which subgroups of leukocytes migrate and what their destinations are. Chemokine receptor expression and activation on malignant cells may be involved in the growth, survival and migration of cancer cells as well as in the tumor vascularisation. CCR7, by binding the chemokines CCL19 and CCL21, is centrally involved in B cell localisation to the secondary lymphoid organs and therefore implicated in lymphadenopathy of various non-hodgkin lymphomas (NHL). In addition to chemokine receptors that have been cloned and described, various orphan receptors with a chemokine receptor-like structure are still not characterized. Atypical, non-signaling chemokine receptors are members of a newly described class of receptors and have been implicated with chemokine clearance and influencing of other signalling receptors. They are consequently considered as potent immuno-modulators and as anti-inflammatory factors and are implicated in progression of cancer. Among these receptors, we are investigating the role of the orphan chemokine (C-C motif) receptor-like 2 (CCRL2), also known as CRAM, a receptor expressed on endothelial cells and B cells in a maturation stage dependent manner, but for which functions and ligands are poorly characterized so far. In an effort to elucidate the role of CRAM and its implication in neoplasias, we have focussed research on identification of ligands and the implication of CRAM in regulating B cell migration in samples from healthy donors and from non-Hodgkin lymphomas.

We characterised the receptor's expression profile by flow cytometry in peripheral blood, bone marrow and lymph node sections of different B cell NHL and correlated it to expression levels of CCR7 and CXCR4. In addition, a screening for ligands was performed using radiolabelled binding assays. The role of CRAM was elucidated using various functional assays, internalisation and transcytosis experiments.

We show that CRAM is an alternative, but non-signaling receptor for the CCR7-activating chemokine CCL19. CRAM is constitutively recycling to and from the cell surface and internalizing the chemokine without degrading it. We found that the receptor is responsible for transcytosis of CCL19 through endothelial cell layers and subsequent presentation, a crucial step in homing of leukocytes to the lymph nodes. On the other hand, when expressed on B cells, CRAM interferes in CCL19 binding to CCR7. We thereby show that CRAM can act as an integrator of different signals, by binding different chemokines and controlling their activity toward surrounding ligands. Chemotaxis experiments demonstrate that CRAM is a negative modulator of CCL19 B cell recruitment. In addition, we have found increased expression in activated B cells, dendritic cells, and also in the B cell malignancies chronic lymphocytic leukemia (B-CLL) and pre-B cell acute lymphoblastic leukemia (pre-B ALL), and are currently evaluating CRAM as a possible prognostic marker in various B-NHLs.

CRAM is a newly identified member of the silent or atypical chemokine receptor group, already known for modulating chemokine availability, together with D6, DARC and CCX-CKR. We have shown here that it contributes to lymphocyte recruitment into peripheral lymphoid tissue by presenting CCL19 on endothelium. It is also involved in CCR7 driven recruitment of B cells by regulating CCL19 availability. Expression of CRAM differs in B cell malignancies for which both CCR7 ligands, CCL19 and CCL21, have already been shown to be implicated in the development of lymphadenopathies. We therefore suggest that CRAM is an additional player and potential biomarker in determining outcome and development of disease.

No relevant conflicts of interest to declare.