lymphoid progenitor

**Hematopoietic lineage position:** Pluripotent hematopoietic stem cell (HSC; CD34+) → multipotent progenitor → common lymphoid progenitor (CLP) vs. common myeloid progenitor (CMP). The CLP is the branch point for the lymphoid lineage.

**Descendants:** - **B cells:** Develop and mature in the bone marrow (pro-B → pre-B → immature B → mature naïve B). Key steps: V(D)J recombination of the heavy chain, then light chain; RAG1/RAG2 and TdT are required. Key transcription factors: E2A, EBF, PAX5. Btk (a tyrosine kinase) is essential for pre-B to B maturation. - **T cells:** CLPs migrate from the bone marrow to the thymus (double-negative CD4–/CD8– → double-positive CD4+/CD8+ → single-positive). They undergo positive selection in the cortex (MHC recognition) and negative selection at the corticomedullary junction/medulla (self-tolerance, AIRE-dependent). Notch1 signaling commits progenitors to the T-cell fate. - **NK cells (innate lymphoid lineage):** CD16+/CD56+, kill virus-infected and tumor cells that downregulate MHC I. Also derived from CLPs, and they depend on IL-15. - Innate lymphoid cells (ILC1, ILC2, ILC3) and some dendritic cells also arise from the CLP.

**Key cytokine and signaling:** - **IL-7** (produced by bone marrow stromal cells and thymic epithelium) drives survival and proliferation of lymphoid progenitors via the IL-7 receptor α chain plus the **common gamma chain (γc)**, signaling through **JAK3**. - γc is shared by the receptors for IL-2, -4, -7, -9, -15, and -21.

**Clinical correlations:** - **X-linked SCID:** Mutation in the IL2RG (common γ chain) gene is the most common SCID form. It causes absent T cells and NK cells and nonfunctional B cells (T–B+NK–). Findings include recurrent infections, thrush, failure to thrive, and absent thymic shadow. Treatment is HSC transplant (or gene therapy). - **JAK3 deficiency:** Autosomal recessive SCID with a similar phenotype (T–B+NK–). - **IL-7Rα deficiency:** T–B+NK+ SCID. - **RAG1/RAG2 mutations (Omenn syndrome / RAG-SCID):** Failed V(D)J recombination leads to absent mature B and T cells (T–B–NK+). Omenn syndrome features erythroderma, eosinophilia, and elevated IgE from a leaky mutation. - **Adenosine deaminase (ADA) deficiency:** Toxic purine metabolites accumulate in lymphocytes, causing T–B–NK– SCID (autosomal recessive). - **Bruton agammaglobulinemia:** Btk mutation causes a block at the pre-B stage, so there are few B cells and low immunoglobulins of all classes. Infections begin after about 6 months, when maternal IgG wanes. - **DiGeorge syndrome (22q11.2 deletion):** Failed thymic development impairs T-cell maturation, but the lymphoid progenitor itself is intact. - **Malignancies:** Transformation of lymphoid progenitors (precursor B or T lymphoblasts) causes **acute lymphoblastic leukemia (ALL)**. This is the most common childhood cancer. Cells are TdT+, CD10+ (CALLA) in B-ALL. T-ALL often presents with a mediastinal mass. t(12;21) carries a good prognosis; t(9;22) carries a poor prognosis.

**High-yield contrast:** The **common myeloid progenitor** gives rise to erythrocytes, platelets, granulocytes (neutrophils, eosinophils, basophils), monocytes/macrophages, and mast cells. Remember that the CLP gives T, B, and NK cells.

**Immunophenotype pearl:** Early lymphoid progenitors express CD34, TdT, and CD10 (B lineage); CD34 and TdT are markers of immature precursors and are lost in mature lymphocytes.