Activation of CD44 induces ICAM‐1/LFA‐1‐independent, Ca2+, Mg2+, —independent adhesion pathway in lymphocyte‐endothelial cell interaction

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Abstract
We have established an endothelial cell line KOP2.16 from pooled mouse lymph nodes. Resting lymphocytes avidly bound to KOP2.16 and migrated underneath the cytoplasm. The binding was partly mediated by VLA‐4 and VCAM‐1, but apparently independent of CD44 since anti‐CD44 antibody examined failed to inhibit the binding. However, pretreatment of lymphocytes with anti‐CD44 resulted in the rapid appearance of Ca2+‐, Mg2+‐independent, LFA‐1/ICAM‐1‐, CD2/LFA‐3,VLA‐4/VCAM‐l‐independent lymphocyte binding, indicating that a novel adhesion pathway was induced by the anti‐CD44 treatment. Interestingly, the elicited adhesion was observed only when anti‐CD44 that block hyaluronate recognition of CD44 were used for lymphocyte pretreatment. Neither hyaluronate itself nor non‐blocking anti‐CD44 up‐regulated the adhesion. Fab fragment of the blocking anti‐CD44 did not induce the up‐regulation unless cross‐linked with a second antibody, indicating that cross‐linking of surface CD44 is necessary for induction of a novel adhesion pathway. We propose that the agonistic anti‐CD44 antibodies induce a novel adhesion pathway by mimicking ligand binding to CD44 on the lymphocyte surface and that non‐hyaluronate ligand(s) is involved in regulation of adhesive function of CD44. Potential involvement of such a regulatory mechanism in lymphocyte homing is discussed.

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