T-cell Development and Normal Subsets:

Early in T-cell development, T-lymphocytes rearrange the T-cell receptor (TCR) genes, leading to expression of the heterodimer TCRαβ or TCRγδ.

TCRαβ represent ~95% of all T-cells and include 2 major subsets T-helper and T-cytotoxic cells.

• Generated from CD4+CD8+ thymocytes undergoing positive selection through interaction of the TCR with self-peptide MHC class I/II  only thymocytes with low-avidity TCRs survive and leave the thymus.
• There are 4 major CD4 and CD8 T-cell maturation stages:
• Naïve
• Central memory
• Effector memory
• Effector cells

Normal T-cell subsets

• Follicular T-helper cells (Tfh)
• CD4+ memory T-cells
• Predominantly in germinal centres where they interact with B-cells
• At least three from: BCL6, PD-1, CXCL13, CD10, ICOS/CD278, CCD5/CD195
• Neoplastic counterpart: angioimmunoblastic T-cell lymphoma and other nodal lymphomas of Tfh origin
• Regulatory T-cells (Treg)
• Immunological tolerance against self and foreign antigens
• Usually 1-2% of circulating CD4+ T-cells
• Expression of CD25high, CD127low, FOXP3
• Neoplastic counterpart: Adult T-cell leukemia/lymphoma (ATLL)
• TCRγδ
• Represent ~5% of lymphocytes
• Predominantly in the spleen, liver and mucosal tissues; role in innate and adaptive immune response
• Increased in autoimmune diseases, inflammatory disorders, infections, other hematological malignancies
• Immunophenotype:
• CD3+, CD4-, CD8-/dim+
• Express all T-cell antigens (CD2, CD3, CD5, CD7)
• Often CD57+, less often CD16+, CD56+; subsets can express CD94
• Reactive changes include complete or partial loss of CD5, dim CD7 and bright CD3 expression.
• Overlapping features with TCRγδ-derived T-cell lymphomas

CD4:CD8 ratio

• Peripheral blood: typically 1-3 for adults, normal range 0.8-6 (Porwit and Béné, 2018)
• Bone marrow: can be lower than in PB, but usually also between 1-3
• Lymph node and tonsils: higher than in PB, with predominance of CD4+ T-cells in reactive states
• Spleen: similar to PB

Increased CD4:CD8 ratio

• Infectious/inflammatory conditions, autoimmune disorders, medication effect
• Consider malignant processes with a reactive T-cell component (e.g. Hodgkin lymphoma with increased CD4+ T-cells)

Decreased CD4:CD8 ratio

• Viral infection (typical in CMV, EBV), autoimmune disease, post-transplantation
• T-cell large granular lymphocytic leukemia; consider if persistent increase in CD57+ T-cells, B-symptoms, cytopenias, organomegaly

Fig 1. CMV reactivation with a CD4:CD8 ratio of 0.15 and increased CD4-/CD8- T-cells

Fig 2. Infectious mononucleosis with a CD4:CD8 ratio of 0.34

Normal/reactive variations:

• Small CD5- and CD7- T-cell subsets are frequent
• Late memory T-cells typically lose CD7
• Increased CD7- subsets in chronic viral infections, inflammatory skin disorders, autoimmune diseases and in the elderly
• Dim CD5 is a feature of CD8+ and TCRγδ subsets
• CD4+CD8+ T-cells normally <1% of peripheral blood T-cells
• Expansion of CD4^bright+/CD8^weak+ T-cells sometimes seen in neoplastic disorders
• Expansion of CD4^weak+/CD8^bright+ typical of infectious mononucleosis

Infectious mononucleosis

• Typical immunophenotype in acute infection: massive expansion of CD2^high, CD7^low, CD38^high, HLA-DR^high, CD28^+/-low, CD45RO^high, CD45RA^-/+low, CD11b^-/+low, CD11c^+/-low, negative for cytotoxic markers

Peripheral blood; lymphocytosis with predominance of T-cells:

Both CD4+CD57+ and CD8+CD57+ subsets are present.

Heterogeneous T-cell populations, no definitive immunophenotypic aberrancies; favor reactive, clinical correlation required, recommend follow-up in 6 months to monitor the evolution. If clinical concern molecular studies for T-cell clonality can be considered.

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