Background.: Subnormothermic machine perfusion shows promise as a viable alternative to static cold storage for prolonged preservation of vascularized composite allografts. To date, most studies have been conducted in nonprimate models, and we report its first utilization in a cynomolgus forearm preclinical model for 24-h preservation. Methods.: Forearms were procured from nonhuman primates weighing between 5 and 8 kg. Two perfusion protocols were implemented. In group 1 (n = 6), limbs were perfused with 1.5 L of recirculating Steen; the perfusate flow was adjusted to maintain stable vascular pressures. In group 2 (n = 6), limbs were perfused with a Steen+ solution, doubling the albumin concentration, with a full-volume exchange at mid-perfusion under a stable low-flow regimen. Dynamic and metabolic parameters, histological analysis, and response to neurostimulation were assessed. Results.: After 24 h of continuous perfusion, mean weight gain was 24.8 ± 5.5% in group 1 and 8.0 ± 4.7% in group 2. Perfusion and metabolic parameters indicated the onset of a compartment syndrome in group 1 with an increase in resistance, lactate, and potassium production at mid-perfusion. In group 2, dynamic and metabolic parameters remained stable for 24 h, and histological analysis showed well-preserved tissues. Finally, in group 2, limbs exhibited muscular contraction recovery quantified at 2.4/5 ± 0.89 after 24 h compared with 0/5 after 3 h of ischemia or after perfusion in group 1. Conclusions.: Successful 24-h perfusion and muscle contractility restoration demonstrate promising outcomes for vascularized composite allograft preservation and reconditioning.

Background: Targeted muscle reinnervation (TMR) was originally developed to enhance prosthetic control in amputees. However, it has also serendipitously demonstrated benefits in reducing phantom pain and neuromas. As a result, it has emerged as a secondary treatment for chronic neuromas in amputees and holds promise for managing neuropathic pain in non-amputee patients, particularly those with neuromas. This review synthesizes the current literature on TMR indications for non-amputee patients, highlighting its potential to address chronic peripheral nerve pain and neuromas beyond its original application in amputation. Methods: A thorough search of the PubMed and Cochrane databases up to January 2024 was conducted following the PRISMA guidelines. Inclusion criteria comprised case series, cohort studies, and randomized controlled trials reporting TMR outcomes in non-amputees. Results: Of 263 articles initially identified, 8 met the inclusion criteria after screening and full-text assessment. The articles were all case series with varied sample sizes and mainly focused on neuroma treatment (n = 6) and neuropathic pain management (n = 2) for both upper and lower extremities. Clinical studies included TMR efficacy for sural nerve neuromas in the lower extremities and hand neuromas, showing pain relief and improved function. Key findings were encouraging, showing successful pain relief, patient satisfaction, and psychosocial improvement, with only rare occurrences of complications such as motor deficits. Conclusions: In non-amputee patients, TMR appears to be a promising option for the surgical management of neuropathic pain, demonstrating favorable patient satisfaction and psychosocial outcomes along with low morbidity rates. Although functional improvements in gait recovery and range of motion are encouraging, further research will be important to confirm and expand upon these findings.

Preclinical and clinical data demonstrate that solid organs differ markedly in their intrinsic antigenicity, inflammatory profiles, and thresholds for tolerogenic conditioning. Intraabdominal organs such as the kidneys and liver are inherently more permissive to tolerance, whereas hearts and lungs remain tolerance-resistant. These organ-specific disparities help understand why chimerism-based protocols that reliably induce renal tolerance often fail in thoracic organs, underscoring the necessity of interpreting tolerance strategies through an organ-specific rather than organ-agnostic lens (3). For instance, kidneys are consistently the most amenable organs for tolerance induction: in both NHP models and haplomatched human recipients, mixed hematopoietic chimerism, often transient, has been sufficient to achieve longterm, immunosuppression-free renal allograft survival (4)(5)(6). In fact, kidneys are often considered to possess a protolerogenic potential, a concept further supported by recent MGH findings showing kidney-induced cardiac allograft tolerance in the NHP model (7). In striking contrast, other solid organs such as the heart (8) and lung grafts remain considerably more refractory, necessitating often stronger immunosuppressive regimens. Cardiac and pulmonary grafts exhibit heightened ischemia-reperfusion injury, stronger innate immune activation, and more proinflammatory tissue-resident leukocyte compartments. These features drive accelerated effector priming, stronger indirect allorecognition, and a limited capacity to sustain donor hematopoietic engraftment, making these organs disproportionately resistant to both chimerism-based and regulatory-cell-based strategies (3). These mechanistic observations underscore that tolerance induction is fundamentally shaped by organ-intrinsic biology, with mixed chimerism proving far more stable and effective in kidneys and liver than in thoracic organs.These disparities become even more pronounced when considering vascularized composite allotransplantation (VCA). High immunosuppressive requirements have, to date, drastically limited the number of VCA procedures performed worldwide (9), and this translates into a particular complexity when applying tolerance-induction strategies to these grafts. VCAs contain multiple, highly antigenic, leukocyte-rich tissues, including skin and mucosa, as well as ischemia-sensitive components such as muscle. This places them at the extreme end of the tolerance-resistance spectrum. In swine, transient mixed chimerism is insufficient to induce full VCA tolerance, and a characteristic split-rejection phenomenon, marked by acceptance of musculoskeletal elements but rejection of the skin, has been consistently observed (10). Achieving stable, multilineage chimerism is required for tolerance of all VCA components; this has only been accomplished through intensified conditioning regimens incorporating augmented irradiation, CTLA4-Ig, anti-IL-6R therapy, and vascularized bone marrow, which enabled long-term tolerance of skin-bearing VCAs across class-I barriers in a clinically relevant model (11). Nonhuman primate data further underscore this divide: prior delayed-tolerance induction protocols in cynomolgus macaques generated robust renal tolerance under identical conditioning yet consistently failed in hand or face VCA models, with early rejection, infectious complications, and absence of chimerism (12). More recently, our group demonstrated, for the first time in the NHP partial face transplant model, that simultaneous tolerance induction can generate transient myeloid and lymphoid chimerism, allowing for prolonged immunosuppression-free survival of a face allograft, although the graft ultimately underwent split and then full rejection (13). Collectively, these findings highlight that VCA immunobiology differs substantially from that of solid organs, cautioning against the direct extrapolation of kidney-derived tolerance strategies to the multi-tissue context of VCA. Furthermore, the extreme sensitivity of these grafts to ischemia-reperfusion injury suggests that they may substantially benefit from ex vivo preservation, preconditioning and reengineering strategies (14), as also highlighted by Blein et al.Taken together, these organ-and species-specific distinctions, further magnified in VCA, underscore that tolerance strategies cannot simply be transferred from one graft type to another. They also outline multiple conceptual layers that shape tolerance-induction research and its clinical translation (Figure 1). Against this backdrop, the authors' effort to synthesize crossorgan tolerance mechanisms and to delineate shared versus organ-specific barriers is both timely and necessary, and their work represents a highly relevant contribution to the field.