Checkpoint Blockade and Acquired Humoral Immune Dysregulation: Emerging Evidence for Antibody Deficiency During Long-Term PD-1/PD-L1 Inhibition

Immune checkpoint inhibitors have transformed the treatment of multiple malignancies by restoring antitumor T-cell activity. Their clinical identity is therefore that of immune-enhancing therapies. However, the biology of the PD-1/PD-L1 axis is more complex than simple immune inhibition. Human inborn errors of PD-1 or PD-L1 signaling indicate that this pathway contributes to immune homeostasis, tolerance, protection against selected infections, and the development of memory B cells and antibody responses. These observations raise an important translational question: Can prolonged pharmacologic blockade of PD-1 or PD-L1 can, in selected clinical contexts, induce or reveal acquired humoral immune dysfunction? This review synthesizes evidence linking PD-1/PD-L1 disruption to altered class-switched memory B-cell biology, antibody responses, vaccine immunogenicity, infection susceptibility, and secondary antibody deficiency. It also incorporates emerging evidence that checkpoint blockade may expand age-associated B cells, a population associated with impaired neutralizing antibody responses after vaccination, and counterbalances evidence that vaccination during ICI therapy may enhance antitumor immunity and survival. Current clinical evidence does not establish the incidence, prevalence, reversibility, dose dependence, or causality of an ICI-induced antibody-deficiency syndrome. Instead, the available data support a hypothesis-generating model of heterogeneous humoral remodeling, ranging from preserved or enhanced vaccine-associated immune activation to qualitative antibody failure and secondary antibody deficiency in susceptible patients. Future studies should incorporate baseline and longitudinal measurements of immunoglobulins, vaccine-specific and neutralizing antibodies, class-switched memory B cells, age-associated B cells, plasmablasts, infection burden, and exposure to immunosuppressive treatment.