Background: Blockade of Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and Programmed Cell Death Protein 1 (PD-1) significantly improves progression-free survival of individuals with cancers, including melanoma. In addition to unleashing antitumor immunity, immune checkpoint inhibition (ICI) therapies disrupt immune regulatory networks critical for maintaining homeostasis in various tissues, including the central nervous system (CNS). Despite growing reports of cancer- and ICI-related cognitive impairments among survivors, our understanding of the pathophysiology of ICI-related neurodegenerative effects is limited.
Methods: In this study, using a murine model of melanoma, cognitive function tests, and neuroimmunological assays, we investigate the cellular mechanisms and impact of combinatorial blockade of CTLA-4 and PD-1 on brain function. Syngeneic melanoma was induced in a C57Bl6 mouse model using D4M-3A.UV2 melanoma cells. After confirmation of tumor growth, cancer-bearing and non-cancer mice received combinatorial treatment of anti-CTLA-4 (two doses per week) and anti-PD-1 (three doses per week) for three weeks. One month after completing ICI treatment, mice were administered learning, memory, and memory consolidation cognitive function tasks. Neuroinflammation, synaptic, and myelin integrity analyses and immune cell status in the brain were conducted to analyze neuroimmunological changes post-ICI treatment.
Results: While tumor-related alterations in brain function were evident, combination ICI specifically disrupted synaptic integrity and reduced myelin levels independent of neurogenesis and neuronal plasticity in both cancer-bearing and non-cancer mice brains. Combination ICI selectively impaired hippocampal-dependent cognitive function. This is associated with two-fold increase in T cell numbers within the brain along with immune activation of myeloid cells, especially microglia. Furthermore, an experimental autoimmune encephalomyelitis model revealed that combination ICI predisposes the CNS to exacerbated autoimmunity, highlighting neuroinflammation-related, and tumor-independent, neurodegenerative sequelae of combination ICI.
Conclusion: Our results demonstrate that combinatorial blockade of CTLA-4 and PD-1 destabilizes neuroimmune-regulatory networks and activates microglia, contributing to long-term neurodegeneration and cognitive impairments. Therefore, selectively limiting microglial activation could be a potential avenue to preserve CNS functions while maintaining the therapeutic benefits of rapidly evolving ICIs and their combinations.
Keywords: Immune checkpoint inhibition; anti-CTLA-4; anti-PD-1; brain; cognitive function; lymphocyte; melanoma; microglia; myelin; neuroinflammation; synaptic loss.