Immune Cell Lineages & CD Surface Markers

The CD system and hematopoietic lineages

Cluster of differentiation (CD). CD numbers name surface molecules defined by monoclonal antibodies. Each lineage wears its own combination, so a flow cytometer loaded with fluorescent anti-CD antibodies can identify and count cell populations.

Stem cells and progenitors. Hematopoietic stem cells are CD34+. They give rise to a common lymphoid progenitor (T, B and NK cells) and a common myeloid progenitor (monocytes/macrophages, granulocytes, mast cells, red cells and platelets). CD45, the leukocyte common antigen, is on essentially all white cells.

Stem-cell harvests are dosed by CD34+ count: G-CSF mobilizes CD34+ cells into blood for collection by apheresis before transplant.

Lymphocyte markers

T cells. Every T cell carries the TCR-CD3 complex. The TCR recognizes peptide bound to MHC, and CD3 transmits the signal through ITAM motifs that recruit the kinase ZAP-70, so CD3 is the pan-T-cell marker. Helper cells are CD4+ and bind MHC class II; cytotoxic cells are CD8+ and bind MHC class I.

Costimulation. Signal 1 (TCR plus CD4 or CD8 on peptide-MHC) is not enough: signal 2 comes from CD28 on the naive T cell binding B7 (CD80/CD86) on the antigen-presenting cell. TCR engagement without CD28 signaling produces anergy. Activated T cells secrete IL-2 and, in CD4 cells, upregulate CD40 ligand (CD40L, CD154), which binds CD40 on B cells (driving class switching) and on macrophages (activating them). CTLA-4 (CD152) binds B7 more avidly than CD28 and turns the response off.

Loss of CD40L causes X-linked hyper-IgM syndrome: B cells cannot switch class, so IgM is normal or high while IgG, IgA and IgE are low.

Regulatory T cells. Tregs are CD4+ CD25+ (CD25 is the α chain of the IL-2 receptor) and require the transcription factor FOXP3. They suppress other lymphocytes with IL-10 and TGF-β; FOXP3 mutation causes IPEX syndrome.

B cells. CD19 and CD20 are pan-B markers (CD20 is the rituximab target). CD21 is complement receptor 2, which binds C3d, and it is the receptor EBV uses to enter B cells. B cells also express CD40, MHC II and B7, so they present antigen to helper T cells. Mature naive B cells display surface IgM and IgD, produced from one VDJ transcript by alternative splicing to the μ or δ constant region.

NK cells. NK cells are CD56+ (NCAM) and CD16+ but CD3-negative, because they have no TCR. CD16 is FcγRIII: it binds the Fc portion of IgG coating a target cell and triggers antibody-dependent cellular cytotoxicity (ADCC) through perforin and granzymes. ADCC is one way anti-CD20 IgG1 antibodies kill lymphoma cells. NK cells also kill cells lacking MHC class I.

Myeloid and antigen-presenting cells

Monocytes and macrophages. CD14 is the co-receptor that hands bacterial LPS to TLR4/MD-2. Macrophages also carry CD40, MHC II and B7, plus receptors for the Fc of IgG and for C3b (CR1, CD35), so opsonized microbes are phagocytosed efficiently.

Langerhans cells. The dendritic antigen-presenting cells of the epidermis: CD1a+, S-100+, with tennis racket-shaped Birbeck granules (formed around langerin, CD207) on electron microscopy. Their clonal proliferation, Langerhans cell histiocytosis, classically causes lytic bone lesions, rash and diabetes insipidus. S-100 also stains melanoma and schwannoma.

Mast cells. Mast cells express CD117 (c-KIT, the receptor tyrosine kinase for stem cell factor) and the high-affinity IgE receptor FcεRI. Allergen cross-linking of bound IgE triggers degranulation. An activating KIT D816V mutation drives systemic mastocytosis. CD117 also stains KIT-mutant gastrointestinal stromal tumors, which respond to imatinib.

Surface molecules as viral receptors

• HIV: gp120 binds CD4 plus a chemokine coreceptor, CCR5 (early, macrophage-tropic strains) or CXCR4 (later, T-tropic strains). Homozygous CCR5Δ32 deletion protects against CCR5-tropic virus, and maraviroc blocks CCR5.
• EBV: gp350 binds CD21 on B cells.
• Rhinovirus: most strains bind ICAM-1 (CD54) on airway epithelium.
• Parvovirus B19: binds the P blood-group antigen on erythroid precursors.

Flow-cytometry patterns in disease

Delayed umbilical cord separation points to LAD1; contrast hypocalcemic tetany (DiGeorge), giant neutrophil granules (Chédiak-Higashi) and eczema with small platelets (Wiskott-Aldrich).