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CD4/CD8 Enumeration

Human immunodeficiency virus (HIV) selectively binds to the cluster determinant 4 (CD4) glycoprotein on the membrane of T cells by its gp120 viral envelope protein. During the initial weeks of infection or during high levels of viremia, CD4 T-cells in gut-associated lymphoid tissue and in the peripheral blood rapidly die. CD4 T-cells may transiently increase weeks after the onset of infection as CD8 cytotoxic T-cells target the virus. However, in untreated patients CD4 counts continue to decrease. The average yearly decrease in CD4 counts is approximately 50 cells/ uL. Enumeration of CD4 and CD8 cells and calculation of the CD4:CD8 ratio is useful in determining a patient’s degree of immunosuppression.

Flow cytometry subclassifies lymphocytes based on immune function. Mature T cells in the peripheral blood express either the CD4 or CD8 antigen. CD4 positive T cells are functionally defined as T helper cells, while CD8 positive cells exhibit either suppressor or cytotoxic activity. Imbalances or deficiencies in the immune system can result from abnormalities in either the CD4 or CD8 population.

HIV-1 infection causes significant changes in the number of CD4 and CD8 positive lymphocytes. Within six months of seroconversion, the CD4 count falls about 30% and the CD8 count increases by 40%, resulting in a decreased CD4/CD8 ratio. HIV positive patients with an absolute CD4 count of <200 cells/uL are classified as having AIDS. There is a direct correlation between the absolute CD4 count and the risk of developing opportunistic infections or malignancy.

 

The goal of antiretroviral therapy is to durably inhibit viral replication so that the patient can maintain an effective immune response to microbial pathogens. The CD4 cell count is the major determinant of initiating therapy.  Most experts recommend starting therapy when the CD4 cell count is between 200 and 350/uL. Therapy may be initiated at higher CD4 cell counts if the viral load is above 50,000 to 100,000 copies/mL or the CD4 cell count is declining by more than 100 cells/uL per year.

In the asymptomatic individual, antiretroviral therapy is evaluated by monitoring CD4 cell count and viral load.  A decrease in viral load indicates reduced viral replication and an increase in CD4 cell count indicates improved immune competence. After initiation of therapy, sequential CD4 cell count measurements are recommended at 4, 8, 12 and 16 to 24 weeks. The CD4 count typically increases by more than 50 cells/uL at 4 to 8 weeks after antiretroviral therapy has been initiated or changed and then increases by an additional 50 to 100 cells/uL per year.  The risk of opportunistic infections is reduced when the CD4 count exceeds 200/uL for 3 to 6 months.  Prophylactic therapy can often be stopped at this time.

The early increase in CD4 cells within the first 8 weeks of therapy is due to redistribution of memory lymphocytes that had been trapped in inflamed lymph nodes into the circulation. Then, there is a slower increase in circulating naive CD4 and CD 8 cells that are necessary for responses to new antigenic challenges. This slow phase persists in most persons for at least the first 2 years of therapy. 

The increase in CD8 positive cells is not unique to HIV-1 infection, since many viruses and vaccinations cause a transient increase in the CD8 population. Generally, the CD4/CD8 ratio is increased in autoimmune diseases and decreased in viral infections. Cyclosporine and prednisone therapies decrease the CD4/CD8 ratio. 

It is important to determine absolute numbers of CD4 and CD8 positive cells, not just the CD4/8 ratio, to distinguish HIV-1 from other viral infections. Critically ill patients without HIV infection may have absolute CD4 counts of less than 300/uL. This lymphopenia is usually transient and resolves following recovery from their illness.

Current clinical guidelines for monitoring patients with HIV infection recommend following only the absolute CD4 count.

Reference Ranges

 

Specimen requirement is one green top and one lavender top tubes of blood.  Specimens cannot be refrigerated.

References

Alexander TS, Human immunodeficiency virus diagnostic testing: 30 years of evolution. Clin Vacc Immunol 2016;23:249-53.

Stein DS et al. CD4+ lymphocyte cell enumeration for prediction of clinical course of human immunodeficiency virus disease: a review. J Infect Dis 1992;165:352.

Fenney C. et al. T-lymphocyte subsets in acute illness. Crit Care Med 1995;23:1680-85.