Neutrophil Interactions with Keratocytes during Corneal Epithelial Wound Healing: A Role for CD18 Integrins

The high incidence of direct corneal injury and increased availability of corrective refractive surgery (e.g., LASIK, PRK), with its accompanying corneal complications, have made clear the need for a better understanding of the intimate events that occur during corneal wound healing.1 Recently, our laboratory reported that neutrophils (PMNs) enter the corneal stroma shortly after epithelial scrape injury, and their presence appears to facilitate wound closure.2 PMN transendothelial migration requires the leukocyte β2 integrin CD18. In injured corneas of CD18−/− mice, both PMN extravasation and wound closure are delayed by 24 hours and 6 hours, respectively.2 Although the precise mechanisms underlying PMN migration in the corneal stroma are poorly understood, it has been suggested that PMN migration within extravascular tissue (i.e., interstitium) is facilitated by integrin-dependent adhesive contacts with the structural elements of the extracellular matrix (e.g., collagen).3 However, migrating PMNs also develop adhesive contacts with resident interstitial cells.4 In the corneal stroma, the primary interstitial cell is the keratocyte, and each keratocyte joins with a neighboring keratocyte to form a cellular network.5 Keratocyte networks lie between the orthogonally arranged collagen layers, extending from limbus to limbus. Our preliminary observations suggested that in response to epithelial scrape injury, migrating PMNs develop close surface contacts with keratocytes,6 raising the possibility that keratocyte networks provide a contact guidance mechanism for PMNs migrating into the injured cornea, functioning as a “cellular highway” during leukocyte trafficking. The purpose of the present study was to use a mouse model of corneal epithelial abrasion to obtain quantitative data on PMN interactions with structural matrix elements (collagen) and resident interstitial cells (keratocytes) during wound healing. In addition, we wanted to examine the “extravascular” role of the leukocyte β2 integrin (CD18) in PMN contacts with collagen and keratocytes. Our results document for the first time that migrating PMNs make extensive surface contacts with keratocytes and that this heterotypic cell interaction requires CD18 integrins.