Low ligand requirement for deletion and lack of synapses in positive selection enforce the gauntlet of thymic T cell maturation

Abstract

Immature double-positive (CD4(+)CD8(+)) thymocytes respond to negatively selecting peptide-MHC ligands by forming an immune synapse that sustains contact with the antigen-presenting cell (APC). Using fluorescently labeled peptides, we showed that as few as two agonist ligands could promote APC contact and subsequent apoptosis in reactive thymocytes. Furthermore, we showed that productive signaling for positive selection, as gauged by nuclear translocation of a green fluorescent protein (GFP)-labeled NFATc construct, did not involve formation of a synapse between thymocytes and selecting epithelial cells in reaggregate thymus cultures. Antibody blockade of endogenous positively selecting ligands prevented NFAT nuclear accumulation in such cultures and reversed NFAT accumulation in previously stimulated thymocytes. Together, these data suggest a "gauntlet" model in which thymocytes mature by continually acquiring and reacquiring positively selecting signals without sustained contact with epithelial cells, thereby allowing them to sample many cell surfaces for potentially negatively selecting ligands.

Figure 1. Number of Peptides Required for Negative Selection in FTOC

(A) Thymi from day 16 5c.c7 Cd74^−/− embryos were cultured for 3 days in the presence of varying concentrations of biotin-MCC. Thymi were then dispersed into single-cell suspensions with trypsin and EDTA, and cells were stained with SA-PE and then anti-I-E^k and analyzed by flow cytometry. I-E^k+ cells were gated and we converted their median PE fluorescence intensity to number of SA-PE molecules using a standard curve generated with QuantiBRITE PE beads (BD Biosciences, r² > 0.99). We used this number to generate a relation between peptide concentration and peptides loaded per APC (number of biotin-MCC per APC = 115 × [biotin-MCC], in μM, r² = 0.979). (B) Thymi from day 16 5c.c7 Cd74^−/− embryos were cultured for 3 days in the presence of varying concentrations of biotin-MCC. Thymocytes were then recovered and stained for CD4 and CD8, and the percentage of surviving DP cells was determined by FACS analysis. The percentage of surviving DP cells is plotted against both concentration of biotin-MCC and the number of biotin-MCC per I-E^k+ cell according to the correspondence shown in part (A). The depicted results represent three independent experiments.

Figure 2. Thymocyte Apoptosis in Response to a Defined Number of Peptide-MHC Ligands

CH27 cells were loaded with biotin-MCC, extensively washed and labeled with SA-PE, and imaged together with 5c.c7 Cd74^−/− thymocytes in the presence of 2 mM Ca²⁺ and Annexin V-FITC. SA-PE signal was acquired within 5 min of interaction in 1 micron sections and then reconstructed with Metamorph software so that the number of SA-PE molecules present could be determined. FITC fluorescence was assessed every 15 min for 6.5 hr. (A and B) A DIC image (A) and PE fluorescence image (B) of a thymocyte-CH27 interaction. PE fluorescence images are rotated 90° so that the thymocyte-APC interface faces the viewer. This interface contains a single peak of PE fluorescence corresponding in fluorescence intensity to a single SA-PE molecule. (C) DIC (top) and Annexin V-FITC fluorescence images (bottom) of this cell couple over time. Frames are taken 30 min apart; the last frame represents 6 hr after interaction. (D and E) A DIC image (D) and PE fluorescence image (E) of a thymocyte-CH27 interaction in which the interface contains a single peak of PE fluorescence with double the intensity of that in (B), corresponding to 2 SA-PE molecules. (F) A time-lapse of DIC and Annexin V-FITC shows that the thymocyte has committed to apoptosis after 1.5–2 hr of interaction (frames are 30 min). (G) Summary of data represented in (A)–(F), n = 12–18 cell couples for each group.

Figure 3. Nuclear Accumulation of Thymocytes’ Endogenous NFATc1 in Positively Selecting Thymi

(A) Thymi were dissected from 5c.c7 B6 mice and thymocytes were liberated directly into 4% paraformaldehyde. Fixed cells were then permeabilized and stained with anti-CD8α PE and mouse anti-NFATc1 and then goat anti-mouse Alexa 488. The left panel shows CD8α staining, the middle panel shows NFATc1 staining, and the right panel shows an overlay with CD8α staining in red and NFATc1 staining in green. (B) 5c.c7 B10.BR thymocytes were isolated and stained as in (A). (C) Summary of data from (A) and (B), and for 5c.c7 B6 thymocytes treated with the ionophore A23187 for 10 min prior to fixation, or thymi allowed to rest in RT PBS for 5 min prior to thymocyte liberation and fixation. Data are expressed as the proportion (mean ± standard deviation) of CD8⁺ cells with nuclear NFAT accumulation (>50% nuclear Alexa 488 staining, n > 300) from two mice for each condition. (D) NFAT transcriptional activity correlates with thymocyte maturation in vivo. 5c.c7 DP thymocytes transduced with NFAT-promoter-driven GFP were injected intrathymically into B10.BR recipients; 5 × 10⁷ thymocytes were prepared from recipient mice at the indicated days after injection, stained for surface expression of CD4 and CD8, and analyzed by flow cytometry. Shown are the CD4-CD8 profiles of total ungated cells (first panel) or cells gated for expression of NFAT-driven GFP on day 2 (second panel), day 3 (third panel), and day 4 (fourth panel) after intrathymic injection. Each profile is representative of two to three mice.

Figure 4. Characterization of the NFATc1-GFP Retrovirus

(A) Video time-lapse microscopy of a mature 5c.c7 T cell expressing NFATc1-GFP encountering a CH27 B cell pulsed with 1 μM MCC. The top row shows DIC, the middle row shows Fura 340/380 ratio in a false-color scale, and the bottom row shows NFATc1-GFP. Times in seconds are given relative to the onset of Ca²⁺ flux above the DIC images, and the ratio of nuclear to cytosolic gfp signal is indicated below the fluorescence images. (B) Video time-lapse microscopy of 5c.c7 DP thymocytes expressing NFATc1-GFP in RTOC with B10.BR thymic stromal cells with no peptide added. The top row shows DIC, and the bottom row shows NFATc1-GFP. Times in minutes are given relative to the onset of imaging, and the ratio of nuclear to cytosolic GFP signal is indicated below the fluorescence images. (C) Summary of video microscopy data, presented as the average number of NFATc1-GFP⁺ cells displaying nuclear accumulation (>50% of total GFP signal confined to the nuclear region in consecutive time points) of NFAT, among >100 cells in each case. For mature T cells, only cells displaying Ca²⁺ flux (fura 2am 340/380 ratio > 1.5) were considered for analysis. RTOC were composed of B10.BR thymic stromal cells unless otherwise indicated.

Figure 5. Paucity of Stable Thymocyte-Thymic APC Contacts in Positively Selecting RTOC

(A) A representative image of an RTOC composed of purified 5c.c7 DP thymocytes expressing CD3ζ-GFP (in green), with unlabeled B10.BR TECs, and PKH26-labeled B10.BR thymic DCs (in red). (B) RTOC generated as in (A), but DCs additionally present MCC-I-E^k. (C) A representative image of an RTOC composed of purified 5c.c7 DP thymocytes expressing CD3ζ-GFP (in green), with unlabeled B6 TECs, and PKH26-labeled B10.BR TECs (in red). (D) RTOC generated as in (C), but TECs additionally present MCC-I-E^k. (E) Summary of data from (A)–(D). Data are expressed as the average percentage of thymocytes with >1.3-fold polarized accumulation of CD3ζ-GFP toward an unlabeled B6 APC (white bars), or >1.3-fold polarized accumulation of CD3ζ-GFP toward a contact with a labeled B10.BR APC (gray bars) for more than 3 min; data are averages (± SD) over three experiments, with >50 cells per experiment. (F–H) Positively selecting ligands are responsible for NFAT mobilization in 5c.c7 thymocytes in selecting RTOC. (F) and (G) are representative images of RTOC composed of 5c.c7 DP thymocytes expressing NFATc1-GFP and B10.BR stromal cells, treated with (F) 20 μg/ml 14.4.4 or (G) 500 μg/ml G35 antibody 4 hr prior to imaging. (H) shows a summary of data from RTOC containing 5c.c7 DP thymocytes expressing NFATc1-GFP and treated with the indicated antibodies. Data are expressed as the average percentage (± SD) of GFP+ thymocytes displaying a sustained (>5 min) nuclear accumulation (>50% nuclear GFP signal) of NFATc1-GFP over three experiments, with n > 50 for each experiment.

Figure 6. Sustained Nuclear NFAT in DP Thymocytes Results from Intermittent, Transient Contact with Selecting Ligands

(A) Time-lapse video microscopy of 5c.c7 DP thymocytes expressing NFATc1-GFP in RTOC with B10.BR stromal cells. RTOCs were allowed to cool to room temperature to quench cell signaling prior to imaging at 37°C. The putatively stimulating epithelial cell is marked by a red asterisk. (B) Time-lapse video microscopy of 5c.c7 DP thymocytes expressing NFATc1-GFP in RTOC with B10.BR stromal cells. We added 20 μg/ml anti-I-E^k antibody 14.4.4 immediately prior to imaging. (C) Percentage of DP thymocytes with nuclear NFATc1-GFP (>50% nuclear GFP signal) over time in selecting RTOC. (D) Percentage of DP thymocytes with nuclear NFATc1-GFP (>50% nuclear GFP signal) over time in RTOC with 14.4.4 added at 0 min. Individual traces in (C) and (D) are separate experiments tracking 12–20 cells each.