T-cell and combined immunodeficiencies — branching viva

Opening question

Examiner: Take me through this infant. What does the low TREC result mean, and what is your frame for managing him? [1]

Candidate: A low T-cell receptor excision circle result on the newborn screen means this infant has few naive T cells leaving the thymus, which raises SCID as the leading diagnosis. My frame is that SCID is a medical emergency — it is fatal within the first one to two years without curative therapy — so I need to confirm the diagnosis with flow cytometry, protect the infant from iatrogenic harm while the diagnosis is secured, and refer urgently to a transplant centre, because the timing of haematopoietic stem cell transplantation — before infection and ideally before 3.5 months of age — is the single most powerful determinant of survival. [1]

Examiner: The family history is interesting. Why does it matter? [1]

Candidate: A maternal uncle who died in infancy before a diagnosis was made is the classic pedigree for X-linked SCID. The IL2RG mutation is carried by the mother, and affected male infants on the maternal line present and die without a diagnosis if newborn screening is not available. This history, combined with the low TREC result and the low lymphocyte count, makes X-linked SCID the leading diagnosis, and flow cytometry is likely to show the T⁻B⁺NK⁻ pattern. [1]

Branch 1 — pathophysiology

Examiner: Explain why loss of T cells is so devastating. Why is it a combined deficiency even when B-cell numbers are normal? [7]

Candidate: T cells are the conductor of the adaptive immune orchestra. Mature CD4⁺ helper T cells provide the signals that every other limb depends on — they express CD40 ligand, which switches B cells from IgM to IgG, IgA, and IgE, and they secrete cytokines that activate macrophages and support CD8⁺ cytotoxic cells. In X-linked SCID, flow cytometry shows B cells — the numbers may look normal — but those B cells cannot class-switch or mature their antibody affinity, because the T-cell help they require is absent. That is why the immunoglobulin levels are profoundly low across all classes, and why the child has infections with bacteria, viruses, and fungi alike. The B cell without its T-cell helper is an instrument without a conductor. [7]

Examiner: What is the molecular defect in X-linked SCID? [7]

Candidate: The common gamma chain, encoded by IL2RG, is a shared subunit of the receptors for six cytokines — IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Its downstream signalling kinase is JAK3. When the gamma chain is absent, T cells and NK cells cannot receive the survival and proliferation signals they need to develop, while B cells are preserved. This produces the T⁻B⁺NK⁻ phenotype. The same phenotype arises, autosomal recessive, from JAK3 deficiency, because JAK3 is the kinase downstream of the gamma chain. So X-linked SCID and JAK3 deficiency share the same flow-cytometry signature. [7]

Branch 2 — immediate protection

Examiner: You have confirmed the flow cytometry shows T⁻B⁺NK⁻. What are the immediate protections you put in place before the transplant? [9]

Candidate: There are four non-negotiable protections. First, stop all live vaccines — BCG, rotavirus, oral polio, and MMR are absolutely contraindicated because the attenuated organisms disseminate fatally in a child without T cells. Second, ensure that every blood product the child receives is irradiated, leucocyte-depleted, and CMV-negative, because non-irradiated blood causes transfusion-associated graft-versus-host disease, which is fatal in SCID. Third, start antimicrobial prophylaxis — co-trimoxazole for Pneumocystis, fluconazole for candidiasis, and aciclovir or ganciclovir if CMV is detected. Fourth, begin immunoglobulin replacement, 400 to 600 milligrams per kilogram every three to four weeks, to provide passive humoral protection, because the child cannot mount antibody responses. [9]

Examiner: He received BCG and rotavirus vaccines at birth, before the screen. What do you do about that? [9]

Candidate: I watch for and treat vaccine-derived disease. The BCG site is inspected for non-healing ulceration, axillary lymphadenitis, or distant lesions — BCGosis — and treated with antimycobacterial therapy directed by infectious diseases. The rotavirus vaccine risk is disseminated rotavirus diarrhoea, so I monitor his stools and hydration. These vaccine-derived complications are sentinel presentations of SCID in countries that immunise at birth, and they worsen the transplant outcome because the active infection is the leading cause of transplant failure. [9]

Branch 3 — the transplant decision

Examiner: Walk me through the transplant — the donor, the conditioning, and the timing. [2]

Candidate: The curative therapy is haematopoietic stem cell transplantation, which replaces the defective immune system with healthy donor stem cells. The donor hierarchy begins with a matched sibling donor, which gives the best outcomes because the cells engraft reliably with minimal graft-versus-host disease. When no matched sibling exists — the common situation given SCID's rarity — the options are a matched unrelated donor from a registry or a haploidentical parent, typically the father, with T-cell depletion to prevent graft-versus-host disease. [2]

The conditioning depends on the donor and the genotype. A matched sibling transplant in a young, infection-free infant may use little or no chemotherapy, because the SCID bone marrow is effectively empty. An unrelated or haploidentical transplant typically uses reduced-intensity or myeloablative conditioning — regimens built around treosulfan, fludarabine, or busulfan — to suppress residual immunity and create space for the donor cells. [10]

The timing is the decisive variable. Transplant before 3.5 months of age and before the onset of infection achieves survival above 90 to 94 percent. Transplant later, or with active infection, drops survival to roughly 70 to 80 percent. This infant is six weeks old and infection-free, so he is in the ideal window — the goal is to transplant him before infection arrives, and the newborn screen is the tool that made that possible. [2] [10]

Examiner: What about gene therapy? [6]

Candidate: Gene therapy is an emerging curative option for specific genotypes, and it is now a real alternative to transplant for X-linked SCID, ADA-SCID, and Wiskott-Aldrich syndrome. A corrected copy of the gene is inserted into the child's own haematopoietic stem cells using a lentiviral or gamma-retroviral vector, and the corrected cells are returned. Because the cells are the child's own, there is no graft-versus-host disease and no need for a matched donor. The early gamma-retroviral trials were complicated by insertional oncogenesis — leukaemia — but the newer lentiviral vectors and the improved safety profiles have made gene therapy an established option in expert centres for selected genotypes. [6] [10]

Branch 4 — the syndromic forms

Examiner: Suppose the flow cytometry had shown a normal or high T-cell count instead. What would that suggest? [7]

Candidate: A normal or high T-cell count in an unwell infant would raise Omenn syndrome, which is a leaky SCID from a hypomorphic RAG mutation. In Omenn, a few autoreactive, oligoclonal T cells escape the defective thymus and attack the patient's own tissues, producing a graft-versus-host-like illness — erythroderma, eosinophilia, lymphadenopathy, hepatosplenomegaly, chronic diarrhoea, and a high IgE. The T-cell count is high, not low, and the disease is often misdiagnosed as severe eczema. The management is the same as classical SCID — protect, treat the skin and infection, and transplant urgently — with the addition that immunosuppression may be needed to control the graft-versus-host-like skin disease before transplant. [7]

Examiner: And DiGeorge syndrome — how is it different from classical SCID? [3]

Candidate: DiGeorge syndrome, from a 22q11.2 deletion, is a developmental defect of the third and fourth pharyngeal pouches, so the thymus does not form properly and T cells cannot mature. The immune deficiency tracks with the degree of thymic hypoplasia — from complete athymia, which needs thymus transplantation, to partial forms with near-normal T-cell counts that need only surveillance. The difference from classical SCID is that DiGeorge is a multisystem syndrome: the infant also has conotruncal cardiac defects, hypocalcaemia from hypoparathyroidism, cleft palate, and characteristic facies. The immunologist's role is to stratify the T-cell deficiency and identify the minority who need thymus transplantation, while the cardiologist, endocrinologist, and speech therapist manage the rest. [3]

Branch 5 — family counselling

Examiner: The genetic testing confirms IL2RG. What do you tell the family about future pregnancies? [6]

Candidate: Confirming an IL2RG mutation identifies the mother as a carrier. For each future pregnancy, there is a one-in-two chance that a male infant will be affected. I would offer maternal carrier testing, sibling screening, and prenatal diagnosis by chorionic villus sampling or preimplantation genetic diagnosis for future pregnancies. The power of early intervention on an affected sibling — transplant before infection — is the difference between near-normal survival and high mortality, so the reproductive counselling is part of the management, not an afterthought. The family that has lost one child to SCID and is offered prenatal diagnosis for the next pregnancy is the family that may see their next child transplanted before infection and survive. [6]

Wrap

Examiner: Summarise the SCID stance in one sentence. [2]

Candidate: Severe combined immunodeficiency is a genetic block in T-cell development that collapses the entire adaptive immune system and is fatal within the first one to two years without curative therapy — so suspect it from lymphopenia and opportunistic infection in a sick infant, confirm with flow cytometry, protect the child from live vaccines and non-irradiated blood, and refer urgently for haematopoietic stem cell transplantation before infection arrives, because transplant before 3.5 months of age achieves survival above 90 percent and newborn screening by TREC is the tool that makes that possible. [2] [10]