Based on respective preclinical data, we hypothesized that repetitive local cell therapy with CAR-NK cells – natural killer (NK) cells, expressing chimeric antigen receptors (CAR) – can modify the immunosuppressive tumor microenvironment to a more immune activated state, further amplified by combined checkpoint inhibition. In contrast, we anticipated minimal systemic effects, resulting in an advantageous safety profile.
 
While CAR-DNA was detected both in blood and cerebrospinal fluid (CSF) during the first days after intracranial CAR-NK cell injection, we found no evidence for long-term engraftment. There were also no relevant changes in blood lymphocyte phenotypes and cytokines, which confirms the advantageous safety profile of local CAR-NK cell therapy. 
The pivotal findings of the immunomonitoring analyses from tissue sampled shortly after combination therapy were (i) an increase in T cell infiltration, and (ii) a decrease of regulatory T cells. We also observed a local immune activation in CSF sampled from the resection cavity. Interestingly, the extent of CD8+ T-cell infiltration in pretreatment tumor tissue correlated with progression-free and overall survival (PFS, OS).

These data provide evidence for the concept of transformation of the immunosuppressive intratumoral microenvironment to a more immune activated condition by combination therapy and support the further development of this approach.