Fig. 1.

Tumor histology of patient P1. At initial diagnosis, H&E stain (a, 200X) shows a high grade glioma with increased cellularity, pleomorphic tumor cells (arrows), increased mitotic figures (arrowheads) and areas of necrosis (stars). Histiocytes are minimal in number as stained by KP-1 (b, 200X), and a high proliferative rate is detected by MIB-1 stain (c, 200X). At 3.5 months after initiating nivolumab treatment, a tumor biopsy shows on H&E stain (D, 200X) a much less cellular lesion with fewer atypical cells and marked histiocytic infiltration highlighted by KP-1 (e, 200X), suggesting reactive changes. MIB-1 stain (f, 200X) shows a much lower proliferative rate index. At 7 months, a new biopsy shows similar findings: on H&E (g, 200X) there is some increase in cellularity and cell atypia, still much less than prior to treatment. KP-1 stain (h, 200X) highlights a large number of histiocytes, and MIB-1 (i, 200X) continues to demonstrate a low proliferative rate index. Scale bar for all panels measures 150 μm