[ADA(+)SCID] as well as ADA deficient [ADA(-)SCID] patients and is probably the treatment of 
choice for this disease (14-23). The overall success rate for bone marrow transplantation in 
SCID is >70%, but ADA(-)SCID responds less well than ADA-normal SCID-(23). Pre- 
transplant cytoablation has usually not been required for HLA-matched marrow transplants for 
SCID because of the lack of host T-cell immunity which might otherwise reject the graft(24). 
Unfortunately, only about 30% of patients will have an HLA-matched sibling donor available 
and until recently transplantation from a mismatched donor resulted in severe and usually fatal 
graft versus host disease (GVHD). Evaluation of ADA(-)SCID recipients of HLA-matched or 
HLA-haploidentical transplants without cytoablation has demonstrated that some patients show 
persistence of donor cells only of .the T cell lineage (24,26). Nevertheless, engraftment of 
donor T-cells (and/or their precursors) alone is sufficient to reconstitute dual system immune 
function in some of these SCID patients. 
Alternative techniques for immune reconstitution of these SCID patients are being developed 
which in some cases may be nearly as successful as matched sibling donor transplantation. The 
development of methods to deplete mature T-cells from the donor marrow has substantially 
eliminated the complication of severe GVHD, previously the principal cause of death in HLA 
non-identical transplants (25). 
In the absence of an HLA-identical sibling donor, T-cell depleted parental bone marrow is 
preferred over an unrelated donor in order to preserve some degree of histocompatibility 
between the engrafted immune system and the host. Such HLA-haploidentical or “half- 
matched” transplantation in SCID has been quite successful in ADA {+) SCID patients (26). 
However, a significant proportion of ADA(-) patients have failed to achieve stable engraftment 
when transplantation protocols lacking ablative conditioning were followed(17). Some of these 
cases have had to be prepared for transplantation by cytoreductive conditioning in order to 
achieve engraftment with T-cell depleted haploidentical marrow (18,19,27). The development 
of the B cell system, antibodies and specific immunity to immunization is delayed, sometimes 
for many months, in the recipients of HLA-haploidentical marrow in comparison with 
recipients of HLA-identical transplants.(28,29) In a recent presentation (Jeffrey Modell 
Foundation, April 19, 1990, in press), Richard O’Reilly, M.D., reported the results of T 
depleted haploidentical bone marrow transplantation in 32 patients with SCID. “Durable 
engraftment” was eventually achieved in 26 patients (81%). However, only 12 of the 
26(46% or just 37.5% of all 32 patients treated with BMT) acheved functional B cell systems 
with this treatment. 
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Recombinant DNA Research, Volume 14 
