Revisiting Co-Stimulation Blockade in Lung Transplantation: Insights from a RAS-Type CLAD Mouse Model

The study by Mineura et al.(1) provides preclinical evidence that early administration of CTLA4-Ig (cytotoxic T-lymphocyte-associated protein-4-immunoglobulin), a first-class T cell co-stimulation blocker, can mitigate fibrotic progression in a mouse model of chronic lung allograft dysfunction (CLAD) with features of restrictive allograft syndrome (RAS). This work is particularly significant, given the paucity of effective therapies for CLAD and more specifically RAS, which portends a dismal prognosis in lung transplant recipients.

In contrast to its established role in kidney transplantation, where it has shown favorable long-term graft survival as a replacement for cyclosporine in a large RCT(2), CTLA4-Ig (belatacept) has not been successfully translated into lung transplantation. A recent pilot clinical trial evaluating belatacept-based immunosuppression following thymoglobulin induction in lung transplant recipients was terminated early due to excess mortality in the belatacept arm(3). This sobering outcome has cast doubt on the safety and efficacy of co-stimulatory blockade for lung transplant recipients. The present study revisits this question in a controlled experimental setting, using a murine lung transplant model. Findings demonstrate that administration of CTLA4-Ig from the day of transplant mitigates CD3+ T cell infiltration in the graft and prevents fibrotic airway remodeling.

One of the notable strengths of this study is the use of a well-characterized murine model that mimics RAS, a less common but highly aggressive phenotype of CLAD. Unlike bronchiolitis obliterans syndrome (BOS), which manifests as small airway obstruction, RAS is characterized by restrictive physiology and extensive parenchymal and pleural fibrosis. The authors’ model recapitulates this pattern through orthotopic BALB/c to B6 lung transplantation with intermittent cyclosporine and methylprednisolone, leading to consistent peribronchial and pleural fibrosis by day 30, hallmarks of RAS-type CLAD. RAS has received less experimental attention than BOS, in part due to its complexity and variable presentation. By focusing on this phenotype, the authors provide much-needed mechanistic insight into a poorly understood form of CLAD resistant to most treatments, adding translational value to their study.

An interesting component of this study is the comparison between preventive (day 0) and therapeutic (day 15) administration of CTLA4-Ig. When given preventively, CTLA4-Ig significantly reduced fibrosis, T cell infiltration, and histologic rejection, consistent with effective early immunomodulation. In contrast, when CTLA4-Ig was administered after the onset of acute cellular rejection (day 15), these benefits were lost. In fact, delayed treatment appeared to reduce intragraft regulatory T cell (Treg) numbers, suggesting that co-stimulation blockade administered in an already inflamed environment might impair immune regulation and even favor rejection. This observation echoes a previous case study showing that conversion from calcineurin inhibitors to belatacept can lead to severe ACR after lung transplantation(4). Collectively, these observations underscore the necessity of carefully timing the initiation of co-stimulation blockade, particularly in the context of lung transplantation. In the clinical setting, this could mean using belatacept for prophylaxis rather than as a treatment for established rejection. However, it is important to note that in the failed pilot trial mentioned above (Huang et al), patients assigned to the belatacept arm also received the co-stimulatory blocker on day 0. While other aspects of the protocol such as thymoglobulin induction and potential over-immunosuppression may help explain the trial’s disappointing results, they serve as a cautionary reminder that timing is not the only factor determining the optimal use of belatacept.

Despite its clinical relevance, the study has several limitations. First, the sample size is modest and rejection was only evaluated 30 days post-transplant. Increasing the number of animals could strengthen the statistical confidence in certain comparisons. Likewise, extending the observation period beyond one month would reveal the lasting efficacy of CTLA4-Ig. Second, the study did not assess donor-specific antibodies (DSA), long suspected contributors to CLAD in humans. Previous work has shown that CTLA4-Ig can prevent DSA development by inhibiting T follicular helper cell activation(5). It would have been particularly interesting to test whether DSA development was affected by CTLA4-Ig in this model. Future experiments incorporating DSA measurements will be needed to clarify this point. Lastly, the authors did not fully identify graft-infiltrating T cells. Specific subsets such as tissue-resident memory CD8+ T cells (Trm) have recently been implicated in chronic rejection and CLAD(6). Whether CTLA4-Ig influenced Trm is an open question that could have been addressed using this experimental model.

Mineura et al. wisely stop short of recommending CTLA4-Ig for clinical use in lung transplant recipients based on the study results alone. However, their findings may pave the way for rational clinical translation. Most importantly, their study suggests that CTLA4-Ig could be safely and effectively used as part of a prophylactic regimen perhaps as a strategy to reduce calcineurin inhibitors and steroid doses. Given the nephrotoxicity and metabolic side effects of long-term calcineurin inhibitor use, CTLA4-Ig could offer a way to minimize these toxicities. Ultimately, this study adds an important layer of mechanistic evidence to the growing literature on co-stimulation blockade in lung transplantation. It supports the hypothesis that early immunologic intervention is essential to prevent irreversible graft damage. It also provides a strong rationale for further research to define the optimal combination strategies for incorporating CTLA4-Ig into lung transplant immunosuppression regimens.