Involvement of avidity for major histocompatibility complex in homeostasis of naive and memory T cells

Abstract

The requirements for survival and self-renewal of peripheral T cells and the nature of mechanisms controlling the size of the naive and memory pool are not completely understood. Here, we examine the involvement of the major histocompatibility complex (MHC) in survival and homeostatic expansion of naive and memory T cells. We show that the homeostatic behavior of naive T cell receptor (TCR)-transgenic T cells can be deduced by the expression levels of TCR and CD5, a negative regulator of TCR signaling. Both these factors determine the strength of TCR stimulation by MHC-derived signals. We further show that, similarly to naive T cells, MHC-derived signals influence the homeostatic expansion capacity of memory T cells under lymphopenic conditions. In contrast to naive T cells, however, memory T cells can reach a homeostatic equilibrium, in which survival/self-renewal of each clone is dissociated from their avidity for MHC-derived signals.

Figure 1.

Homeostatic expansion capacity of naive TCR-transgenic T cells and TCR and CD5 density. (A) Comparison of lymphopenia-induced cell division, and expression levels of TCRβ, IL-7Rα, and CD5 in naive A18 (top row), A1 (middle), and AND T cells (bottom row). A18 and A1 T cells were analyzed for their CFSE profile at day 14 after transfer into syngeneic Rag1 ^−/− hosts, while for AND T cells day 7 is shown. Levels of TCRβ, IL-7Rα, and CD5 in TCR-transgenic T cells (filled histograms) are compared with those in polyclonal T cells from A mice (open histograms). Numbers within the histogram plots represent the mean fluorescence intensity (MFI) of 4–5 mice of each strain. (B) IL-2 production (as % of maximal) in culture supernatants by TCR-transgenic T cells in response to the indicated amount of plate-bound anti-CD3. Numbers within the plots represent the mean concentration of anti-CD3 (ng/ml) that led to half the maximal IL-2 production in 3–4 mice of each strain.

Figure 2.

Effect of CD4 overexpression on homeostatic behavior of naive A18 T cells. (A) Gated TCR⁺ 4A18 T cells can be further subdivided into CD8⁻ and CD8⁺, referred to as CD4 and CD8 4A18 T cells, respectively. Numbers within the quadrant represent the mean percentage of CD4 and CD8 4A18 T cells in splenic T cells. Histogram plots compare TCRβ, CD4, and CD5 levels between CD4 (top row) or CD8 (bottom row) 4A18 T cells (filled histograms) and the original A18 T cells (open histograms). (B) IL-2 production (fluorescence units) in culture supernatants by naive A18 (○) and 4A18 T cells (♦) in response to the indicated amount of C5p. (C) Absolute number of splenic T cells in the original A18 mice (○) and 4A18 T cells, either total (♦) or separated as CD4 (▵) and CD8 (▾). Each point represents an individual mouse (P < 0.03 between A18 and either CD4 or CD8 4A18). (D) CFSE profiles of CD4 (top row) and CD8 4A18 T cells (bottom row) at day 7 after transfer into syngeneic Rag1 ^−/− hosts.

Figure 3.

Comparison between naive A1 T cells homozygous (A1^hom and heterozygous (A1^het) for the TCR transgene. (A) TCRβ and CD5 levels assessed by flow cytometry on A1^hom (open histograms) and A1^het T cells (filled histograms). (B) Comparison of TCR density (MFI; P < 0.0001), % of T cells in peripheral blood (P = 0.021), and absolute number of peripheral splenic and lymph node T cells (P = 0.011) between A1^hom (○) and A1^het T cells (♦). Each point represents an individual mouse.

Figure 4.

Survival/homeostatic expansion capacity of naive TCR-transgenic T cells as a function of TCR and CD5 density. Absolute number of splenic T cells in 5–12 mice from A18 (•), 4A18 (▿), A1^het (▪), A1^hom (⋄), or AND (▴) TCR-transgenic strains plotted against the CD5 density (MFI) (A), or TCR density (MFI) minus the CD5 density (MFI) of each T cell clone (B). The data in B are plotted according to a simple two-variable regression equation: Y = a + b₁X₁ + b₂X₂, where Y is the absolute number of splenic T cells and X₁ and X₂, the TCR and CD5 MFI, respectively. The regression coefficients were calculated as a = –0.6299, b₁ = 0.0221, and b₂ = –0.005, by SigmaPlot for Windows (SPSS Inc.).

Figure 5.

Survival/homeostatic expansion capacity of different naive TCR-transgenic T cell clones after cotransfer. (A) Percentage of A1^hom (open bars) and A1^het T cells (gray bars) in total T cells in peripheral blood, after cotransfer of naive A1^hom and A1^het T cells into syngeneic Rag1 ^−/− female hosts (n = 5). The ‘week 0’ time point represents the percentage at day 2 after transfer (P = 0.024, week 0 vs. week 8 and P = 0.011, week 0 vs. week 10). Identical results were obtained from secondary lymphoid organs at the end of the experiment. (B) Percentage of A18 (gray bars), CD4 4A18 T cells (open bars) in total peripheral blood T cells, after cotransfer of naive A18 and 4A18 T cells into syngeneic Rag1 ^−/− hosts (n = 3). One of two experiments with similar results is shown.

Figure 6.

Effect of MHC-derived signals on homeostatic behavior of memory T cells. (A) CFSE profiles of memory A1 T cells 7 d after transfer (3 × 10⁶ memory A1 T cells per recipient) into syngeneic (H2^a), allogeneic (H2^b or H2^q), and MHC-deficient (H2⁻) secondary recipients. (B) CFSE profiles of memory A1 (top) and AND T cells (bottom) 7 d after cotransfer into syngeneic H2^a secondary recipients. (C) Absolute number of memory A1 (♦) and AND T cells (○) recovered from the spleen of secondary syngeneic H2 ^a Rag1 ^−/− recipients, after cotransfer of memory A1 and AND T cells, in 1:1 ratio, or transfer of memory A1 T cells alone (▵). Each time point represents the mean number of cells from the spleen of 2–3 recipient mice. The dashed line denotes the injected number of cells of each type. (D) Percentage of memory A1 (gray bars) and AND T cells (open bars) in total peripheral blood T cells of the recipients described in C. Each time point is the average of 2–5 mice. Similar results were obtained with either H2 ^a Rag2 ^−/− Il2rg ^−/− (H2^a) or H2 ^a Rag1 ^−/− recipients in additional experiments.

Figure 7.

Apparent lack of competition between different memory T cell clones. (A) Naive A1^hom and A1^het T cells were cotransferred together with HY-pulsed syngeneic DCs into syngeneic H2^a (n = 5) recipients and the percentage of A1^hom (open bars) and A1^het T cells (gray bars) in total peripheral blood T cells over time is shown. (B) Naive A18 and 4A18 T cells were cotransferred into syngeneic H2^a (n = 7). Recipients were immunized with a C5p/LPS intravenously injection. The percentage of A18 (gray bars) and CD4 4A18 T cells (white bars) in total peripheral blood T cells over time is shown. Note that the time scale is in weeks up to week 3 and in alternate weeks for the rest of the experiment.