Absence of Macrophage Inflammatory Protein-1α Prevents the Development of Blinding Herpes Stromal Keratitis

Prior studies in our laboratory have suggested that the CC chemokine macrophage inflammatory protein-1α (MIP-1α) may be an important mediator in the blinding ocular inflammation which develops following herpes simplex virus type 1 (HSV-1) infection of the murine cornea. To directly test this hypothesis, MIP-1α-deficient (−/−) mice and their wild-type (+/+) counterparts were infected topically on the scarified cornea with 2.5 × 10⁵ PFU of HSV-1 strain RE and subsequently graded for corneal opacity. Four weeks postinfection (p.i.), the mean corneal opacity score of −/− mice was 1.1 ± 0.3 while that of the +/+ mice was 3.7 ± 0.5. No detectable infiltrating CD4⁺ T cells were seen histologically at 14 or 21 days p.i. in −/− animals, whereas the mean CD4⁺ T-cell count per field (36 fields counted) in +/+ hosts was 26 ± 2 (P < 0.001). In addition, neutrophil counts in the −/− mouse corneas were reduced by >80% in comparison to the wild-type controls. At 2 weeks p.i., no interleukin-2 or gamma interferon could be detected in six of seven −/− mice, whereas both T-cell cytokines were readily demonstrable in +/+ mouse corneas. Also, MIP-2 and monocyte chemoattractant protein-1 protein levels were significantly lower in MIP-1α −/− mouse corneas than in +/+ host corneas, suggesting that MIP-1α directly, or more likely indirectly, influences the expression of other chemokines. Interestingly, despite the paucity of infiltrating cells, HSV-1 clearance from the eyes of −/− mice was not significantly different from that observed in +/+ hosts. We conclude that MIP-1α is not needed to control virus growth in the cornea but is essential for the development of severe stromal keratitis.