IS-free Treg HaploHCT

For MAC regimen: Total Myeloid and Lymphoid Irradiation (TMLI) delivered through Radiation Oncology institutional standards and comprised of 13.5 Gy TMI (9 fractions, 1.5 Gy per fraction, 2 fractions per day) and 11.7 Gy TLI (9 fractions, 1.3 Gy per fraction, 2 fractions per day). OR Total Body Irradiation (TBI) comprised of 12 Gy (6 fractions, 2 Gy per fraction, 2 fractions per day) For RIC regimen: TBI comprised of 2 Gy in 1 fraction.

For MAC regimen: 30 mg/m\^2/d in 100 ml normal saline (NS) will be administered as a bolus infusion administered by IV infusion over approximately 30 minutes for 5 days (on day -10, -9, -8, -7, -6) For RIC regimen: 40 mg/m\^2/d in 100 ml NS will be administered as a bolus by IV infusion over approximately 30 minutes for 4 days (on day -10, -9, -8, -7)

For MAC regimen: 3.75 mg/kg diluted in NS to a final concentration of 1mg/mL will be administered by IV infusion over approximately 4 hours daily for 2 days (on day -10, -9) For RIC regimen: 5 mg/kg diluted in NS to a final concentration of 1mg/mL will be administered by IV infusion over approximately 4 hours twice daily for 1 day (on day -11)

For MAC regimen only: 15 mg/kg diluted in NS per institutional standard and will be administered by IV over 1 hour or as directed per institutional standard practice, daily on D-8, -7.

For MAC regimen only: 3.75mg/kg (25% of cyclophosphamide dose) diluted in 50 mL NS and administered IV over 30 min, will be infused starting 30 min prior to cyclophosphamide and for 3 doses thereafter, at 3, 6 and 9h after cyclophosphamide

Target 'Treg-enriched' cell dose is 1-2 x 10\^6 cells/kg. Cells will be given intravenously on Day -4. The day -1 calculated unmodified PBMC T ('Teff') cell dose will be adjusted to maintain a targeted cell ratio of 2 'Treg-enriched' cells:1 'Teff' cell.

Unmodified donor PBMCs will be infused at a calculated 'Teff' dose of 1x10\^6 CD3+ T cells/kg, adjusted per the caveats below: A) If the 'Treg-enriched' product infused was at target dose of 2x106 cells/kg but had ≥30% CD4+CD25+CD127hi cells, the unmodified 'Teff' (calculated) cell dose infused on day -1 will be halved to 0.5x10\^6 CD3+ T cells/kg. B) If the 'Treg-enriched' product was in the range of 1-2x106 cells/kg, the unmodified 'Teff' (calculated) cell dose on day -1 will be adjusted to between 0.5-1x10\^6 CD3+ T cells/kg, dosed to maintain a 2 'Treg-enriched':1 'Teff' (calculated) ratio of infused cells. C) If the 'Treg-enriched' product infused met both caveats A and B above, the unmodified 'Teff' (calculated) cell dose infused on day -1 will be halved to 0.5x10\^6 CD3+ T cells/kg.

The megadose donor CD34+ PBSC infusion target is \>10x10\^6 CD34+ cells/kg (ABW or IBW, whichever is greater). If the CD34+ graft has \<6x10\^6 CD34+ cells/kg (ABW or IBW, whichever is greater) (below megadose minimum) it WILL be infused in order to rescue recipient hematopoiesis, and the patient will remain on study.

For RIC regimen only: 100 mg/m2 will be administered as a bolus by IV infusion over approximately 30 minutes for 1 day (on day -6)