TNFRSF9 and COVID-19: Ingenuity Pathway Analysis of transcripts with increased expression in cluster 0 from COVID-19 patients with severe relative to mild illness showed significant enrichment of transcripts in multiple co-stimulation pathways (OX40, CD27, CD28, 4-1BB, CD40), the NF-κB and apoptosis signaling pathways (Fig. 4B-D, fig.