ISG15 and infection: The selected genes with the most positive regression coefficients, corresponding to higher expression in the Definite group, were GNLY, encoding an antibacterial peptide present in cytolytic granules of cytotoxic T cells and natural killer cells (26); SLC38A2, encoding a glutamine transporter upregulated in CD28-stimulated T cells (27, 28); FFAR3, encoding a G protein–coupled receptor activated by short-chain fatty acids that is induced by alveolar macrophages upon infection (29); and the interferon-stimulated genes PSMB8, ISG15, and IRF1 (Figure 2C and Supplemental Table 2).