PF4 and infection: For example, on the third day post infection, increased expression of neuronal activation marker c-fos protein (51), increased cerebellar gene expression of programmed cell death proteins 1 (PD-1), lymphocyte activation gene 3 (LAG-3) and T lymphocyte-associated protein 4 (CTLA-4), and reduction of hippocampal chemokine ligand 4 (CXCL-4) occur (50).