Cancers often evade immune surveillance by adopting peripheral tissue–tolerance mechanisms, such as the expression of programmed cell death ligand 1 (PD-L1), the inhibition of which results in potent antitumor immunity. Here, we show that cyclin-dependent kinase 5 (Cdk5), a serine-threonine kinase that is highly active in postmitotic neurons and in many cancers, allows medulloblastoma (MB) to evade immune elimination. Interferon-γ (IFN-γ)-induced PD-L1 up-regulation on MB requires Cdk5, and disruption of Cdk5 expression in a mouse model of MB results in potent CD4+ T cell–mediated tumor rejection. Loss of Cdk5 results in persistent expression of the PD-L1 transcriptional repressors, the interferon regulatory factors IRF2 and IRF2BP2, which likely leads to reduced PD-L1 expression on tumors. Our finding highlights a central role for Cdk5 in immune checkpoint regulation by tumor cells.

After traumatic spinal cord injury, functional deficits increase as axons die back from the center of the lesion and the glial scar forms. Axonal die back occurs in two phases: an initial axon intrinsic stage that occurs over the first several hours and a secondary phase which takes place over the first few weeks after injury. Here, we examine the secondary phase, which is marked by infiltration of macrophages. Using powerful time lapse multi-photon imaging, we captured images of interactions between Cx3cr1+/GFP macrophages and microglia and Thy-1YFP axons in a mouse dorsal column crush spinal cord injury model. Over the first few weeks after injury, axonal retraction bulbs within the lesion are static except when axonal fragments are lost by a blebbing mechanism in response to physical contact followed by phagocytosis by mobile Cx3Cr1+/GFP cells. Utilizing a radiation chimera model to distinguish marrow-derived cells from radio-resistant CNS resident microglia, we determined that the vast majority of accumulated cells in the lesion are derived from the blood and only these are associated with axonal damage. Interestingly, CNS-resident Cx3Cr1+/GFP microglia did not increasingly accumulate nor participate in neuronal destruction in the lesion during this time period. Additionally, we found that the blood-derived cells consisted mainly of singly labeled Ccr2+/RFP macrophages, singly labeled Cx3Cr1+/GFP macrophages and a small population of double-labeled cells. Since all axon destructive events were seen in contact with a Cx3Cr1+/GFP cell, we infer that the CCR2 single positive subset is likely not robustly involved in axonal dieback. Finally, in our model, deletion of CCR2, a chemokine receptor, did not alter the position of axons after dieback. Understanding the in vivo cellular interactions involved in secondary axonal injury may lead to clinical treatment candidates involving modulation of destructive infiltrating blood monocytes.