Assessment of HIV-specific T cells | HIV Management Guidelines

In addition to measuring the blood count or proportion of total CD4+ T cells or CD4+ T cell subpopulations, it is also possible to assess the proportion of blood T cells that specifically respond to HIV antigens. Whilst this type of test is not used in the routine clinical setting, it has utility in research settings for understanding immunological responses against HIV and particularly in assessment of HIV vaccine candidates. Table 2 summarise the assays used to identify HIV-specific T cells.

Table 2. Assays used to detect HIV-specific T cells

Assay	CD4	CD8	Quantitative	Functional	Interpretation
Tetramer binding, using flow cytometry ¹	√²	√	√		Number of T cells recognising speciﬁc HIV antigens
ELISPOT assay ³	√	√	√	√	Number of T cells capable of producing cytokines following recognition of HIV antigens
Intracellular cytokine production, using flow cytometry ⁴	√	√	√	√	Number of T cells capable of producing cytokines following recognition of HIV antigens
Cr⁵¹ release assay⁵		√	√	√	Number of T cells able to kill cells expressing HIV antigens (CTLs)
Degranulation assay, using flow cytometry⁶		√	√	√	Number of T cells able to degranulate following recognition of HIV antigens
Lymphoproliferation assay⁷	√			√	Number of CD4+ T cells able to proliferate following recognition of HIV antigens
^1.Tetrameric HLA-peptide complexes detect T cells that express a T cell receptor complex which speciﬁcally recognises a given peptide presented by a given human leukocyte antigen molecule. ^2.Less developed for CD4+ T cells as tetramers harder to create, specific CD4+ T cell frequency lower and reduced MHC class II and T cell receptor affinity. ^3.T cells are exposed to HIV antigens. Cells recognising these antigens produce cytokines such as interferon-gamma (IFN-gamma). Secreted IFN-gamma is captured by speciﬁc antibodies coating the well. Cells are then removed by washing and bound IFN-gamma is detected by colorimetric techniques. ^4.T cells are exposed to HIV antigens. Cells recognising these antigens produce cytokines such as IFN-gamma. Cell membranes are then made permeable to permit ﬂuorescent monoclonal antibodies to enter cells, which bind to the cytokine of interest. The cytokine-containing cells are then enumerated by ﬂow cytometry. ^5.Autologous B cells are transformed to express HIV antigens on their cell surface. These cells are pulsed with 51Chromium (51Cr) which remains intracellular. CD8+ T cells are then mixed with autologous B cell targets. HIV-speciﬁc cytotoxic T lymphocytes lyse target cells and thus release 51Cr which can then be measured. ^6.T cells are exposed to HIV antigens. CD8+ T cells recognising these antigens degranulate, which can be detected by CD107a expression on cell surface by flow cytometry. ^7.Lymphoproliferative assay – CD4+ T cells are mixed with HIV antigens and 3H-thymidine. CD4+ T cells which recognise HIV antigens proliferate and incorporate 3H-thymidine. The stimulation index reported compares the amount of 3H-thymidine incorporated into cells exposed to HIV antigens compared with those exposed to control antigens.
ELISPOT: enzyme-linked immunospot assay CTL: cytotoxic T lymphocyte.