Therapeutic blockade of the PD-1/PD-L1 immune checkpoint is central to current cancer immunotherapy because it can reverse T cell exhaustion and restore antitumor responses. However, interfering with this regulatory axis can carry the unintended consequence of altering host control of chronic or latent infections. The role of PD-1/PD-L1 signaling in granulomatous infectious diseases, including chronic actinomycetoma caused by Nocardia brasiliensis, has not been well characterized. This study used a murine model to determine whether disrupting PD-1/PD-L1 interactions affects immune containment and disease progression in an established chronic bacterial infection.
Researchers used BALB/c mice harboring established chronic actinomycetoma due to Nocardia brasiliensis. To interrogate the contribution of PD-1/PD-L1 signaling to infection control, animals with chronic lesions were treated with the anti–PD-L1 monoclonal antibody atezolizumab. Outcomes assessed included survival, clinical progression, lesion histopathology, bacterial burden, and characterization of systemic versus local immune responses using cytokine measurements and flow cytometry of lymphoid organs and lesion tissue.
Histologic assessment of actinomycetoma lesions revealed a paucicellular lymphocytic infiltrate overall. Among the limited lymphocytes present, CD8+ T cells showed high expression of PD-1, suggesting an exhausted or regulated phenotype localized within the chronic lesions. This local pattern prompted evaluation of whether releasing PD-1/PD-L1–mediated inhibition would restore effector function and improve bacterial clearance.
Contrary to the expectation that checkpoint inhibition might enhance antimicrobial immunity, treatment with atezolizumab was associated with worsened outcomes in this model. Specifically, PD-L1 blockade correlated with increased mortality, a loss of granuloma integrity, and exacerbated tissue necrosis within lesions. Survivors treated with anti–PD-L1 exhibited higher bacterial loads compared with untreated controls. The study notes that the precise cause of death in treated animals was not directly established.
Importantly, these adverse histopathologic and microbiologic changes occurred without the classical clinical signs of sepsis reported in this work, indicating that mortality and lesion progression were not accompanied by overt systemic septic presentation detectable by the study assessments.
Analysis of systemic cytokines did not demonstrate elevations in hallmark proinflammatory mediators often associated with severe infection such as IL-6 and TNF-α following PD-L1 blockade. Instead, the blockade elicited a paradoxical cytokine profile characterized by elevations in IL-12p70 and IL-10. This mixed pattern suggests complex immune modulation rather than a simple shift toward heightened systemic inflammation.
Although checkpoint inhibition altered cytokine patterns and coincided with worse bacterial containment and tissue damage, it did not convert the lesion microenvironment into a lymphocyte-driven or macrophage-mediated clearance setting. The local microenvironment remained dominated by neutrophils and was not effectively reprogrammed by PD-L1 blockade into one that promoted bacterial eradication. Authors interpret these observations to mean that intact PD-1/PD-L1 signaling contributes to maintaining an immune equilibrium that supports granuloma architecture and bacterial containment in this chronic infection model.
The authors identify methodological limitations, most notably the absence of an isotype control group for the antibody treatment. Additionally, while increased mortality was observed in treated animals, the study did not directly establish the proximate cause(s) of death. Those points limit the ability to fully attribute all observed effects to PD-L1 blockade and constrain mechanistic inference.
In this BALB/c mouse model of chronic Nocardia brasiliensis actinomycetoma, therapeutic PD-L1 blockade with atezolizumab was associated with severe immunopathology, loss of granuloma integrity, higher tissue necrosis, increased bacterial burden in survivors, and reduced survival. The absence of classic sepsis signs and lack of increases in IL-6 and TNF-α, combined with elevations in IL-12p70 and IL-10, underscore a complex and paradoxical immunologic response rather than straightforward systemic hyperinflammation.
These findings support the interpretation that the PD-1/PD-L1 axis plays a role in maintaining immune balance and containment of bacteria in chronic actinomycetoma in this experimental system. Clinically, the results highlight potential risks of immune checkpoint inhibition in patients with underlying chronic granulomatous or localized infections; however, translation to human disease requires caution because of species differences and study limitations noted by the authors.
Given the methodological caveats and the focused nature of the model, further work with appropriate controls and mechanistic interrogation is required before generalizing these results to clinical practice or to other chronic infectious diseases.