Immunotherapy with oncolytic herpes simplex virus-1 therapy offers an innovative, targeted, less-toxic approach for treating brain tumors. However, a major obstacle in maximizing oncolytic virotherapy is a lack of comprehensive understanding of the underlying mechanisms that unfold in CNS tumors/associated microenvironments after infusion of virus. We demonstrate that our multiplex biomarker screening platform comprehensively informs changes in both topographical location and functional states of resident/infiltrating immune cells that play a role in neuropathology after treatment with HSV G207 in a pediatric Phase 1 patient. Using this approach, we identified robust infiltration of CD8+ T cells suggesting activation of the immune response following virotherapy; however there was a corresponding upregulation of checkpoint proteins PD-1, PD-L1, CTLA-4, and IDO revealing a potential role for checkpoint inhibitors. Such work may ultimately lead to an understanding of the governing pathobiology of tumors, thereby fostering development of novel therapeutics tailored to produce optimal responses.