Deficient Severe Combined Immunodeficiency Research Articles

Adenosine-deaminase-associated immunodeficiency. I. Differential sensitivities of lymphocyte subpopulations exposed to 2-deoxycoformycin in vivo.

In order to obtain a better understanding of the degree of immune dysfunctions caused by the absence of adenosine deaminase, we gave a single i.p. injection of 2'-deoxycoformycin (2-dcf), a potent inhibitor of the enzyme ADA at various doses into adult Syrian hamsters. These animals were examined for their ability to mount primary in vivo antibody responses to helper T cell dependent (Th-d) and helper T cell independent (Th-ind) antigens. Hamsters treated with 0.5 mg/kg of 2-dcf mounted enhanced splenic plaque-forming cell (PFC) responses to sheep erythrocytes, a Th-d antigen, and to pneumococcal polysaccharide type III (SIII), a Th-ind antigen. Treatment of animals with 1.0 mg/kg of 2-dcf resulted in a significantly depressed (P less than 0.001) PFC response to Th-d antigen, but a further enhanced response to Th-ind antigen. One mechanism which may be responsible for such a dichotomous response to these two types of antigens was selective dysfunction of T cell subpopulations. At higher doses (1.5-4.0 mg/kg), PFC responses to both types of antigens were significantly suppressed. Immunoenhancement at low doses of 2-def was attributed to an increased susceptibility of T suppressor cells to 2-dcf. This hypothesis was confirmed by priming the 2-dcf-treated animals with low-dose Th-ind antigens. These animals failed to induce low-dose tolerance by stimulation of antigen-specific suppressor T cell subsets. At low doses, B cells and T helper cell functions were found to be intact, as further confirmed by priming the animals with the carrier keyhole limpet haemocyanin (KLH) and challenging with trinitrophenyl-KLH. This dose-dependent selective susceptibility of various T cell subpopulations and B cells may explain the heterogeneity of clinical, biochemical and immunological parameters observed in children with ADA deficiency severe combined immunodeficiency.

ZAP-70 Restoration in Mice by In Vivo Thymic Electroporation

Viral and non-viral vectors have been developed for gene therapy, but their use is associated with unresolved problems of efficacy and safety. Efficient and safe methods of DNA delivery need to be found for medical application. Here we report a new monopolar system of non-viral electro-gene transfer into the thymus in vivo that consists of the local application of electrical pulses after the introduction of the DNA. We assessed the proof of concept of this approach by correcting ZAP-70 deficient severe combined immunodeficiency (SCID) in mice. The thymic electro-gene transfer of the pCMV-ZAP-70-IRES-EGFP vector in these mice resulted in rapid T cell differentiation in the thymus with mature lymphocytes detected by three weeks in secondary lymphoid organs. Moreover, this system resulted in the generation of long-term functional T lymphocytes. Peripheral reconstituted T cells displayed a diversified T cell receptor (TCR) repertoire, and were responsive to alloantigens in vivo. This process applied to the thymus could represent a simplified and effective alternative for gene therapy of T cell immunodeficiencies.

Reconstitution of immune function in adenosine deaminase deficient severe combined immunodeficiency following hematopoietic stem cell gene therapy

Reconstitution of immune function in adenosine deaminase deficient severe combined immunodeficiency following hematopoietic stem cell gene therapy

IL-7 receptor deficient SCID with a unique intronic mutation and post-transplant autoimmunity due to chronic GVHD

Severe combined immunodeficiency (SCID) may result from a variety of genetic defects that impair the development of T cells. Signaling mediated by the cytokine interleukin-7 is essential for the differentiation of T cells from lymphoid progenitors, and mutations of either the interleukin-7 receptor α chain (IL-7Rα) or its associated cytokine receptor chain, the common γ chain (γc), result in SCID. Here we report a case of SCID due to heterozygous mutations of the IL7R gene encoding IL-7Rα. A previously unrecognized mutation found within intron 3 created a new exon between exons 3 and 4 in the mRNA transcribed from this allele, producing a truncated, unstable mRNA. This mutation illustrates the necessity of evaluating both coding and non-coding regions of genes when searching for pathogenic mutations. Following hematopoietic stem cell transplantation of our patient, immune reconstitution was accompanied by two unusual complications, immune-mediated myositis and myasthenia gravis.

Unusual clinical and immunologic manifestations of transplacentally acquired maternal T cells in severe combined immunodeficiency

The persistence of transplacentally transferred maternal T cells is common in infants with severe combined immunodeficiency (SCID), occurring in more than half of patients with SCID undergoing transplantation at our institution. These T cells respond poorly to mitogens in vitro but can cause cutaneous graft-versus-host disease; however, other effects of these cells are unknown. We describe 2 infants with SCID who had unusual problems associated with transplacentally transferred maternal T cells. Patient 1 was a 5-month-old girl with Janus kinase 3-deficient SCID who had 4% circulating CD3(+) T cells but no lymphocyte proliferative response to mitogens. Although the number of T cells increased after 2 nonchemoablated, T cell-depleted, haploidentical, paternal bone marrow transplantations, T-cell function failed to develop, and she became pancytopenic. Restriction fragment length polymorphism studies of flow cytometry-sorted blood T cells revealed all to be of maternal origin. A subsequent nonchemoablated, T cell-depleted maternal transplantation resulted in normal T-cell function and marrow recovery. Patient 2 was a 9-month-old girl with IL-7Ralpha-deficient SCID who presented with autoimmune pancytopenia. She had 8% blood T cells (all CD45RO(+)) but no response to mitogens. High-resolution HLA sequence-specific priming typing detected both maternal haplotypes, indicating the presence of maternal cells. Her pancytopenia resolved after treatment with rituximab and was thought to be due to host B-cell activation by transplacentally acquired maternal T cells. Persistent transplacentally acquired maternal T cells in infants with SCID can mediate immunologic functions despite failing to respond to mitogens in vitro. We present evidence that these cells can cause allograft rejection and immune cytopenias.

Tumor Necrosis Factor Receptor 2–Mediated Tumor Suppression Is Nitric Oxide Dependent and Involves Angiostasis

Tumor necrosis factor (TNF) binds to two different receptors. Although most of its functions are attributed to TNF receptor 1 (TNFR1), the independent role of TNFR2 is still largely unknown. Using TNFR single or double knock-out mice, we show here that the expression of TNFR2 alone on host cells was sufficient to suppress the growth of TNF-secreting tumors in both immune competent and T/B lymphocyte-deficient severe combined immunodeficiency (SCID) mice. Histologic studies showed that TNF recruited, via TNFR2, large numbers of macrophages and efficiently inhibited angiogenesis in the tumor. In vitro, TNF activated TNFR1-deficient macrophages to produce nitric oxide (NO). Treatment of TNFR1 knock-out mice with L-NAME, a specific NO synthase inhibitor, almost completely eliminated TNF-induced angiostasis and tumor suppression. Moreover, L-NAME acted only during the first few days of tumor growth. Our results show for the first time that TNFR2 expressed on host innate immune cells is sufficient to mediate the antitumor effect of TNF, and NO is necessary for this process, possibly by inhibition of angiogenesis in the tumor.

Immunodeficiency Is A Tough Nut to CRAC: The Importance of Calcium Flux in T Cell Activation

Severe Combined Immunodeficiency (SCID) is a rare primary immunodeficiency disease often characterized by a block in T cell development, which may also affect the normal development of B cells and NK cells. Several different mutations are known to give rise to SCID, and multiple genes are involved. Consequently, there are several different forms of SCID, which can be classified according to the metabolic and cellular defects that impede normal lymphocyte function. The two most prevalent forms of SCID are X-linked SCID and adenosine deaminase (ADA) deficiency SCID, together accounting for approximately 70-80% of disease cases. Other genetic abnormalities associated with this syndrome range from defective T cell receptor rearrangement to non-functional signaling molecules. Recently, a new genetic defect has been described in which mutations in a key component of Ca(2+) release activated-channels (CRAC) result in T lymphocyte malfunction.

Perspectives of gene therapy for primary immunodeficiencies

Standard therapies for patients with severe primary immunodeficiencies include bone marrow transplantation and, for adenosine deaminase deficiency, enzyme replacement. In the last decade, gene therapy has been developed as an alternative for these conditions. We summarize the recent advances in gene therapy for primary immunodeficiencies and discuss the unexpected occurrence of leukemia in a gene therapy trial for X-linked severe combined immunodeficiency. Eight of 10 infants with X-linked severe combined immunodeficiency who received autologous hematopoietic stem cells transduced with a retroviral vector carrying the IL2RG complementary DNA achieved immune reconstitution. However, the two youngest patients developed leukemic expansions of gene-corrected cells. The first case had proliferation of a gamma delta T cell clone, and the second case had three alpha beta T cell clones derived from a single transduced progenitor. Leukemic cells in both patients aberrantly expressed the LIM domain only-2 transcription factor due to retroviral vector insertions in this locus. After receiving anti-leukemic treatment one patient achieved a lasting remission, but the other relapsed. Four adenosine deaminase deficient severe combined immunodeficiency patients also developed functional immunity after receiving autologous hematopoietic stem cells transduced with the adenosine deaminase gene complementary DNA following submyeloablative chemotherapy. Chronic granulomatous disease, Wiskott-Aldrich syndrome, JAK3 deficiency and RAG2 deficiency are other immunodeficiencies being studied as candidates for gene therapy. Gene therapy is a promising therapeutic option for some primary immunodeficiencies, especially when cells expressing the correct gene have a selective advantage. More clinical trials with closer patient monitoring are under way to define which patients may benefit from this approach, and strategies are being developed to understand and ultimately reduce the risk of leukemia secondary to retroviral vector insertion.

Restoration of B Cell Development after Lentiviral Transduction of CD34+ Hematopoietic Cells from RAG-1 Deficient Patient.

Patients lacking expression of either RAG-1 or RAG-2 suffer from a Severe Combined Immuno-Deficiency (SCID) disease characterized by an early block in T and B lymphocytes differentiation leading to the absence of both mature lymphocyte subsets. This disease accounts for about 20% of SCID and the only curative treatment is hematopoietic stem cell transplantation, usually successful when an HLA-genoidentical donor is available. In the absence of such a donor, the success rate decreases along with the degree of HLA disparity between donor and recipient. Ex-vivo gene therapy of hematopoietic stem cells can be considered as an alternative treatment as a selective advantage of transgene-expressing cells is expected. Moreover, constitutive expression of only one of the two RAG proteins should not be harmful as concomitant expression of both genes is required for the recombination activity.We used a lentivecteur containing the RAG-1 cDNA transgene as a therapeutic vector to transduce bone marrow CD34+ cells obtained from RAG-1 deficient patients. The transduced cells were injected into N0D-SCID mice previously irradiated (3Gy) and treated with an anti-TMb1 antibody. Ten weeks after transplantation, in all treated mice, 35±15% of the bone-marrow cells express the human CD45 marker. In this population, 24±2% co-express CD19 and IgM demonstrating that B cell differentiation capacity has been restored. We also detected some CD33+ cells attesting the presence of human myeloid progenitors cells. Altogether, these results suggest that both lymphoid and myeloid precursors have been transduced and demonstrate that gene transfer into hematopoietic cells can reconstitute B cell development in vivo.Our data support the hypothesis that gene therapy could represent a possible alternative to bone marrow transplantation in RAG-1 deficient SCID disease.

Gene therapy for severe combined immunodeficiency

Severe combined immunodeficiency disorders are ideal candidates for gene therapy. Current conventional treatments have significant associated toxicities and often result in incomplete immune reconstitution. The definition of the molecular pathophysiology of the diseases, together with the prediction that gene corrected cells have significant growth and survival advantage has led to clinical gene therapy studies. Clear therapeutic benefits have been demonstrated in patients with adenosine deaminase deficient severe combined immunodeficiency and X-linked severe combined immunodeficiency. Serious complications have arisen, however, and further longitudinal research to quantify benefit and risk is required. Improved and targeted gene deliveries are future goals, but for some primary immunodeficiencies, gene therapy is set to become the standard treatment.

CORRECTION OF ADA-SCID BY STEM CELL GENE THERAPY COMBINED WITH NONMYELOABLATIVE CONDITIONING

Aiuti A, Slavin S, Aker M, et al. Science. 2002;296:2410–2413 To use improved gene therapy techniques to correct adenoside deaminase (ADA)-deficient severe combined immunodeficiency (SCID). Two patients (7 months old and 2.5 years) with ADA-SCID who lacked an HLA-identical sibling donor and for whom polyethylene glycol conjugated (PEG)-ADA was unavailable. Patients underwent collection of autologous CD34+ cells (stem cells) from bone marrow that were …

Pathways of removal of free DNA vector ends in normal and DNA-PKcs-deficient SCID mouse hepatocytes transduced with rAAV vectors.

Elucidation of the mechanisms of transformation of single-stranded (ss) recombinant adeno-associated virus (rAAV) vector genomes into a variety of stable double-stranded (ds) forms is key to a complete understanding of rAAV vector transduction in vivo. Ds monomer genome formation and cellular ds DNA break (DSB) repair pathways that remove free vector ends toxic to cells, presumably play a central role in this process. By delivering rAAV and naked ds linear DNA vectors into livers of DNA-dependent protein kinase catalytic subunit (DNA-PKcs)-deficient severe combined immunodeficiency (SCID) and wild-type mice, we demonstrate the presence of three major pathways for free ds vector end removal: (1) DNA-PKcs-dependent self-circularization, (2) DNA-PKcs-independent self-circularization, and (3) DNA-PKcs-independent concatemerization. By using the DNA-PKcs-independent pathways, mouse hepatocytes efficiently removed free ds rAAV vector ends even in the absence of DNA-PKcs. Our studies suggest a hierarchical organization of these processes; self-circularization is the preferred pathway over concatemerization, although the former has a limited capacity to remove free vector ends. These studies shed new light on the molecular mechanisms of rAAV vector transduction in vivo.

931. Correction of ADA-SCID by Stem Cell Gene Therapy Combined with a Non-Myeloablative Conditioning

Aiuti A, Slavin S, Aker M, et al. Science. 2002;296:2410–2413 To use improved gene therapy techniques to correct adenoside deaminase (ADA)-deficient severe combined immunodeficiency (SCID). Two patients (7 months old and 2.5 years) with ADA-SCID who lacked an HLA-identical sibling donor and for whom polyethylene glycol conjugated (PEG)-ADA was unavailable. Patients underwent collection of autologous CD34+ cells (stem cells) from bone marrow that were …

Effects of metabolic disorders on the brain: Can these effects be reversed with bone marrow transplantation?

Effects of metabolic disorders on the brain: Can these effects be reversed with bone marrow transplantation?

Cognitive and behavioral abnormalities in adenosine deaminase deficient severe combined immunodeficiency

Objective: The objective was to evaluate the cognitive, behavioral, and neurodevelopmental function in patients with adenosine deaminase deficient severe combined immunodeficiency (ADA-SCID) and to compare the findings with those of a case control group of patients without ADA-SCID. Study design: Case-matched pairs of patients with ADA-SCID (n = 11) and patients without ADA-SCID who had undergone bone marrow transplantation were recruited. Subjects were assessed by age-appropriate standard tests of intelligence, behavior, and neurodevelopment. Results: Cognitive ability was not significantly different between the 2 groups, but patients with ADA-SCID showed a significant inverse correlation between deoxyadenosinetrisphosphate levels at diagnosis and IQ (P =.048). Behavioral assessment showed that patients with ADA-SCID functioned in the pathologic range on all domains, whereas mean scores for the control group were within normal limits. Behavioral impairment in patients with ADA-SCID also showed a significant positive correlation with age (P =.026). Conclusions: Cognitive function in ADA deficiency is adversely affected by the severity of metabolic derangement at the time of diagnosis. In addition, patients with ADA-SCID have significant behavioral abnormalities after transplantation. These defects are not due to the transplant procedure but reflect the systemic nature of ADA deficiency. These findings have important implications for future medical and nonmedical management strategies. (J Pediatr 2001;139:44-50)

Full hematopoietic engraftment after allogeneic bone marrow transplantation without cytoreduction in a child with severe combined immunodeficiency

AbstractBone marrow transplantation (BMT) for severe combined immunodeficiency (SCID) with human leukocyte antigen (HLA)-identical sibling donors but no pretransplantation cytoreduction results in T-lymphocyte engraftment and correction of immune dysfunction but not in full hematopoietic engraftment. A case of a 17-month-old girl with adenosine deaminase (ADA) deficiency SCID in whom full hematopoietic engraftment developed after BMT from her HLA-identical sister is reported. No myeloablative or immunosuppressive therapy or graft-versus-host disease (GVHD) prophylaxis was given. Mild acute and chronic GVHD developed, her B- and T-cell functions became reconstituted, and she is well almost 11 years after BMT. After BMT, repeated studies demonstrated: (1) Loss of a recipient-specific chromosomal marker in peripheral blood leukocytes (PBLs) and bone marrow, (2) conversion of recipient red blood cell antigens to donor type, (3) conversion of recipient T-cell, B-cell, and granulocyte lineages to donor origin by DNA analysis, and (4) increased ADA activity and metabolic correction in red blood cells and PBLs.

Granulocyte colony-stimulating factor recruitment of CD34+ progenitors to peripheral blood: impaired mobilization in chronic granulomatous disease and adenosine deaminase--deficient severe combined immunodeficiency disease patients

Peripheral blood (PB) CD34+ cells mobilized by granulocyte colony- stimulating factor (G-CSF) administration are potentially useful for transplantation and as a target of gene transfer for therapy of hematopoietic disorders. Efficient harvest and planning for clinical use of PB CD34+ cells ideally requires foreknowledge of the expected mobilization kinetics and yield. We developed a sensitive flow cytometric assay for accurately enumerating CD34+ cells throughout the range seen at baseline to peak mobilization. We used this assay to assess the kinetics of G-CSF-mediated mobilization of CD34+ cells to PB in normal volunteers and in patients with chronic granulomatous disease (CGD) or adenosine deaminase (ADA)-deficient severe combined immunodeficiency disease (SCID). Two dose levels of G-CSF were examined (5 and 10 micrograms/kg/d for 7 days). Both doses were well tolerated. For normal subjects and patients an increase in PB CD34+ cells was first detected only preceding the third dose of G-CSF (day 3), peaked transiently on day 5 or 6, and then decreased thereafter despite additional doses of G-CSF. With 32 normal volunteers mean peak CD34+ cell counts were 57 and 76 cells/mm2 of blood (5 and 10 micrograms doses, respectively), whereas for 18 CGD patients the mean peaks were 31 and 40 cells/mm2 of blood. For 2 ADA-deficient SCID patients studied at a G-CSF dose of 5 micrograms/kg/d, the average peak was 16 cells/mm2 of blood. For both of these patient groups mobilization of CD34+ cells to PB was impaired compared with similarly treated normal subjects (P < .05). By contrast to the kinetics of the CD34+ cell mobilization, the absolute neutrophil count (ANC) increased markedly by 6 hours after the first dose of G-CSF and then increased steadily through day 8. At days 5 and 6 (peak mobilization of CD34+ cells) the mean ANC of CGD and ADA patients was only slightly lower ( < or = 15%) than that seen with normal subjects, whereas the difference in CD34+ cell mobilization was > 48%. Thus, ANC is not a reliable surrogate to predict peak PB CD34+ cell counts and direct enumeration of PB CD34+ counts should be undertaken in decisions regarding timing and duration of apheresis to harvest a specific number of these cells. Finally, unexpected, but significant differences in the PB CD34+ cell mobilization between normal subjects and patients with inherited disorders can occur and underscores the importance of establishing the expected mobilization of PB CD34+ cells in the planning of treatment approaches using these cells.

Granulocyte Colony-Stimulating Factor Recruitment of CD34+ Progenitors to Peripheral Blood: Impaired Mobilization in Chronic Granulomatous Disease and Adenosine Deaminase-Deficient Severe Combined Immunodeficiency Disease Patients

Peripheral blood (PB) CD34+ cells mobilized by granulocyte colony-stimulating factor (G-CSF) administration are potentially useful for transplantation and as a target of gene transfer for therapy of hematopoietic disorders. Efficient harvest and planning for clinical use of PB CD34+ cells ideally requires foreknowledge of the expected mobilization kinetics and yield. We developed a sensitive flow cytometric assay for accurately enumerating CD34+ cells throughout the range seen at baseline to peak mobilization. We used this assay to assess the kinetics of G-CSF-mediated mobilization of CD34+ cells to PB in normal volunteers and in patients with chronic granulomatous disease (CGD) or adenosine deaminase (ADA)-deficient severe combined immunodeficiency disease (SCID). Two dose levels of G-CSF were examined (5 and 10 micrograms/kg/d for 7 days). Both doses were well tolerated. For normal subjects and patients an increase in PB CD34+ cells was first detected only preceding the third dose of G-CSF (day 3), peaked transiently on day 5 or 6, and then decreased thereafter despite additional doses of G-CSF. With 32 normal volunteers mean peak CD34+ cell counts were 57 and 76 cells/mm2 of blood (5 and 10 micrograms doses, respectively), whereas for 18 CGD patients the mean peaks were 31 and 40 cells/mm2 of blood. For 2 ADA-deficient SCID patients studied at a G-CSF dose of 5 micrograms/kg/d, the average peak was 16 cells/mm2 of blood. For both of these patient groups mobilization of CD34+ cells to PB was impaired compared with similarly treated normal subjects (P < .05). By contrast to the kinetics of the CD34+ cell mobilization, the absolute neutrophil count (ANC) increased markedly by 6 hours after the first dose of G-CSF and then increased steadily through day 8. At days 5 and 6 (peak mobilization of CD34+ cells) the mean ANC of CGD and ADA patients was only slightly lower ( < or = 15%) than that seen with normal subjects, whereas the difference in CD34+ cell mobilization was > 48%. Thus, ANC is not a reliable surrogate to predict peak PB CD34+ cell counts and direct enumeration of PB CD34+ counts should be undertaken in decisions regarding timing and duration of apheresis to harvest a specific number of these cells. Finally, unexpected, but significant differences in the PB CD34+ cell mobilization between normal subjects and patients with inherited disorders can occur and underscores the importance of establishing the expected mobilization of PB CD34+ cells in the planning of treatment approaches using these cells.

HUMAN SEVERE COMBINED IMMUNODEFICIENCY (SCID): GENETIC, PHENOTYPIC AND FUNCTIONAL DIVERSITY IN 95 INFANTS. • 41

Severe combined immunodeficiency (SCID) is a rare, fatal syndrome of diverse genetic etiology characterized by profound deficiencies of T and B cell function. In the U.S., it affects primarily male infants. Some are due to mutations in the genes encoding adenosine deaminase (ADA), the common gamma chain (Yc) of the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors, or Janus kinase 3 (Jak3). However, for a significant proportion of SCID's, the molecular basis remains unknown. In an effort to identify unique phenotypic and/or functional features that could help identify fundamental causes, we analyzed the clinical, genetic and immunologic features of 95 SCID infants who presented consecutively to this institution between August of 1965 and December of 1995. Eighty (84%) were male and 15 (16%) female; 74 (78%) were white, 13 (14%) black, and 8 (8%) Hispanic. The 95 infants fit 7 categories: 16 (17%) were ADA deficient; 6 (6.3%) had Jak3 deficiency; 16 (17%) had autosomal recessive inheritance of unknown molecular cause; 1 (1.2%) had reticular dysgenesis; 1 (1.2%) had cartilage hair hypoplasia; 38 (40%) had X-linked SCID; and 17 (18%) (all males) had SCID of unknown type. Numbers of total lymphocytes and the various lymphocyte phenotypes distinguished the groups: ADA deficient SCID's had the lowest numbers of total lymphocytes; B cells were present in highest number and NK cells in lowest number in X-linked and Jak3-deficient SCIDs. Mean numbers of NK cells were highest in autosomal recessive and unknown types of SCID. All types were profoundly deficient in T cells. NK function was tested in 60 and found to be normal in 25. Although the family history was positive in only 34% of X-linked cases, it is likely that as many as two-thirds of the boys with SCID have mutations in Yc. The presence of NK function in ADA deficient, autosomal recessive, and some SCID's of unknown type, and abnormally low NK numbers and function in a majority of X-linked and Jak3-deficient SCID's implies that some molecular lesions affect T, B and NK cells (Yc and Jak3 mutations), but that others affect only T cells (ADA deficiency), and others T and B cells (possible recombinase abnormalities). The findings in this group of SCID's, the largest reported in the U.S., argue against a common lineage for T and NK cells and may provide clues as to the molecular bases of SCID in the 35 (37%) infants who did not have X-linked SCID, Jak3 deficiency or ADA deficiency.

Donor type natural killer cells after haploidentical T cell-depleted bone marrow stem cell transplantation in a patient with adenosine deaminase-deficient severe combined immunodeficiency

T cell-depleted haploidentical (parental) bone marrow stem cell transplants are given to most infants with the syndrome of severe combined immunodeficiency (SCID) because they have no available HLA-identical sibling potential donors. Since they usually do not undergo cytoreduction prior to transplantation, these children later demonstrate mixed hematopoietic chimerism. Most often, T cells (but usually not B lymphocytes, macrophages, or other hematopoietic cells) can be shown to be of donor type. The origin of natural killer (NK) cells in such chimeras has not been reported. Two lymphocyte lines derived from the CD16 + fraction of an adenosine deaminase (ADA)-deficient male SCID's blood mononuclear cells (MNC) 13 months following maternal marrow stem cell transplantation demonstrated typical phenotypic and functional characteristics of NK cells after expansion. Karyotyping showed both lines to be XX. Thus, NK cell engraftment can occur in SCID infants who have not been conditioned, even when significant NK cell function is present before transplantation.