**7. Non-HLA antibodies**

The most pertinent immunologic barriers in solid organ transplantation are HLA and ABO incompatibility however; rejection episodes are reported in the absence of DSA raising the possibility of non-HLA antibodies.

Several non-HLA antibodies against epithelial and endothelial proteins are associated with poor renal graft outcomes [79]. Angiotensin II type I receptor antibodies have attracted much attention [80]. Non-HLA antibodies are not part of the immunologic risk assessment currently however; they are worth consideration in some instances.

### **8. Outcomes of desensitization in cardiac and lung transplantation**

Desensitization protocols in cardiac and lung transplantation are based on experience from renal transplantation. As in renal transplantation, variability in the desensitization protocols, clinical endpoints, and antibody measurement techniques also exist in cardiac and lung transplantation [81]. Most cardiac and lung desensitization studies are small, have shorter follow-up periods, and report inconsistent results. Protocols vary between centers, but most comprise apheresis, IVIG, and rituximab combinations. Desensitization in cardiac and lung transplantation is conducted in a deceased donor setup, considering the unique characteristics of the individual patient and the nature of the organ being transplanted. Some centers commence desensitization before transplantation, while others administer it in the perioperative period. Below is a brief account of a few pertinent studies.

In a case-control study, thirty-five sensitized (PRA > 10%) cardiac transplant candidates were desensitized with five sessions of PE (1.5 plasma volume) and 20-gram IVIG administered after every PE. All patients underwent cardiac transplantation. Seventeen had positive CDCXM before desensitization. Negative crossmatch was achieved in seven cases. Both class I and class II DSA declined significantly posttreatment. Survival was better than the control group; however, more rejections were observed in the intervention group [81].

In another study (n=16), sensitized cardiac transplant candidates (PRA >10%) were treated with a single session of PE (1.5 plasma volume with albumin + four units FFP replacement) and 20-gram IVIG followed by triple maintenance immunosuppression regimen [82]. PRA declined from 55.5 to 34.9%. Survival and rejection rates at 22 months were similar among the intervention and control groups.

In a large observational study, 21 sensitized (PRA > 10%) cardiac transplant candidates were desensitized with plasma exchange, 2 g/kg IVIG, and rituximab (375 mg/kg2 in refractory cases) [83]. Mean PRA declined from 70.5 to 30.2% postdesensitization. Five-year survival was 71.4% in the desensitized group compared to 81.1% in the untreated sensitized and 75.7% in the unsensitized group. ABMR

rates were higher in the desensitized group (66.7%, 89.2% and 96.5%, respectively). Vasculopathy rates were similar between the groups.

In a small study, 16 sensitized left ventricular assist device (LVAD) recipients were treated with IVIG and a single dose of cyclophosphamide. Class I HLA DSA declined by 33%, but the DSA reappeared at four weeks [84]. In another study, a weekly 500 mg /kg IVIG dose reduced PRA in three LVAD recipients to 23% at one month and 52% at two months [85].

Eighteen highly sensitized lung transplant candidates (PRA ≥ 80%) were desensitized with methylprednisolone, bortezomib, rituximab, IVIG, and PE [86]. Nine patients were transplanted; two were on the waiting list at the time of publication, while seven patients were removed from the waiting list due to medical reasons. This protocol failed to reduce pretransplant PRA.

Another retrospective study desensitized fifty-three lung transplant candidates in the perioperative period with PE, IVIG, and anti-thymocyte globulin [87]. The sensitized cohort had preformed DSA, but only five had positive CDCXM. Clinical outcomes were like the unsensitized cohort.

### **9. Conclusion**

Highly sensitized patients face longer waiting times on the transplant list than unsensitized patients. They experience greater morbidity and mortality. Sensitized patients have limited access to compatible organs. There is an unmet medical need to expand their access to transplantation.

Efforts have been made to circumvent incompatible transplants in sensitized patients, e.g., utilizing potential living donors, increasing the chances of well-matched organs *via* priority points in the regular allocation, acceptable match program, and kidney-paired donation. Although successful to a certain extent, such measures have failed to provide compatible organs to all sensitized patients. Desensitization could facilitate access to transplantation in selected sensitized patients who fail to access compatible organs.

It is worth noting that the outcomes of desensitization are inferior to compatible transplants. However, compared to dialysis, incompatible transplantation is cost-effective, enhances quality of life, and confers survival benefit. Acute and chronic ABMR risk is high in incompatible transplantation and is a significant cause of allograft loss. Infections and malignancies are frequent due to higher cumulative doses of immunosuppressive drugs.

Implementing novel and innovative strategies to avoid crossing the immunological barriers is vital. Transplantation tolerance has the potential to minimize sensitization. HLA epitope matching is a step forward in lowering post-transplant sensitization. Newer biomarkers are needed for immune monitoring and early diagnosis of graft rejection. Running multiple desensitization randomized trials is not feasible due to limited resources and finite eligible patients. Innovative trial designs such as master protocols could offer testing multiple therapeutics in one trial, thus accelerating the development of effective protocols and therapies. Moreover, standardizing HLA antibody and cPRA measurements will be a significant achievement in this field.

### **Acknowledgements**

This work is not funded.
