Regulation of the affinity of the β2 integrin LFA-1 by chemokines is critical to lymphocyte trafficking, but the signaling mechanisms that control this process are not well understood. Here we investigated the signaling events controlling LFA-1 affinity triggering by chemokines in human primary T lymphocytes. We found that the small GTPase Rac1 mediated chemokine-induced LFA-1 affinity triggering and lymphocyte arrest in high endothelial venules. Unexpectedly, another Rho family member, Cdc42, negatively regulated LFA-1 activation. The Rho effectors PLD1 and PIP5KC were also critical to LFA-1 affinity modulation. Notably, PIP5KC was found to specifically control the transition of LFA-1 from an extended low-intermediate state to a high-affinity state, which correlated with lymphocyte arrest. Thus, chemokines control lymphocyte trafficking by triggering a Rho-dependent signaling cascade leading to conformer-specific modulation of LFA-1 affinity.
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