Contrasting Patterns of Alloreactivity Amongst Malignant and Nonmalignant Diseases Receiving Haploidentical PBSC GRAFT and Post-Transplant Cyclophosphamide

Haploidentical donor is often the only source of graft for most patients in developing countries due to lack of suitably matched donors from international registries and the prohibitive cost involved in the procurement process. We conducted a pilot study with posttransplant cyclophosphamide and haploidentical PBSC graft. The donors selected were either mother or NIMA mismatched siblings irrespective of NK cell alloreactivity. The conditioning protocol was developed based on Johns Hopkins regimen of Fludarabine and low-dose Cyclophosphamide pre-transplant with escalating dose Melphalan 70-140 mg/m2 replacing 2 Gy TBI. Post-transplant Cyclophosphamide was administered 72hrs after infusion of the graft at 50 mg/kg twice at 24 hrs interval followed by Cyclosporine and MMF. 
8 patients (median age-16, 5-43) underwent Haplo-HCT; 5 patients with refractory AML had a median BM blast count of 50% (20-80%) having failed at least two lines of treatment. Two patients had severe aplastic anemia and one had thalassemia. The ones with AML received high dose AraC and mitoxantrone from day-14to -12. The conditioning was tolerated without any major non-hematological toxicity. The median CD34 was 7.06 x 10⁶/kg (range 5.05-11.06) and CD3 was 36 x 10⁷/kg (range 8-79). 
All patients engrafted with neutrophils > 500/µl on day +14 (range12-17) and platelet count > 20,000/µl on day +15 (range 9-38) with > 95% donor chimerism on day +30 with morphological CR.  None of the patients with leukemia developed de-novo GVHD. Three relapsed between days 100-150 and two of them achieved a CR following a second transplant from the same donor. Two patients in CR died of multi-drug resistant gram-negative bacterial sepsis. All three patients with non-malignant disease developed unexpected alloreactivity. One patient with SAA developed severe refractory HLH on day +21 and the other a periengraftment idiopathic pneumonitis. Both succumbed to their complications. Another patient with thalassemia developed severe HLH on day 60 related to EBV, whilst on treatment for grade 3 acute GVHD. In multiply treated advanced leukemia, the introduction of the PBSC graft without any immunosuppression for 72 hours  probably allowed  a strong GVL effect and post-transplant cyclophosphamide was successful in abrogating clinically significant GVHD. In sharp contrast, all the three patients with non-malignant disease experienced early and unexpected alloreactivity with the same protocol. We speculate that the lack of previous cytotoxic therapy might have left them vulnerable to such alloreactivity mediated by residual host APCs.