Lung tissues were removed from CD8-alpha knock-out mice six days post-intranasal inoculation of 1610 6 PFU cHV68. Tissues were enzymatically digested and endothelial cells purified using the endothelial cell marker CD31 (PECAM-1) (Fig. S4A) . Briefly, cells were stained with the following antibodies: anti-CD31-biotin (PECAM-1, clone MEC 13.3, rat IgG2a k) (BD Bioscience), anti-CD45-PE (B220, clone RA3-6B2, Rat IgG2a k) (BD Bioscience), anti-F4/80-PE (clone BM8, Rat IgG2a k) (eBioscience, San Diego, CA), anti-CD4-PE (L3T4, clone RM4-5, Rat IgG2a k) (eBioscience), anti-CD8a-PE (Ly-2, clone 53-6.7, Rat IgG2a k) (BD Bioscience), and anti-Ly-6G and Ly-6c-PE (Gr-1, clone RB6-8C5, Rat IgG2b k) (BD Bioscience). Cells were magnetically labeled with Anti-Biotin MultiSort beads, separated with Octo-MACS separation unit, and released from the MultiSort beads as per manufacturer's instructions (Miltenyi Biotec Inc., Auburn, CA). Released cells were incubated with Anti-PE MultiSort beads and depleted of contaminating PE positive cells by magnetic separation. Flow cytometric analysis was performed to confirm flow through fraction from PE column as enriched in CD31 positive cells and depleted of PE positive cells (Fig. S5B) .