Modification of the FoxP3 transcription factor principally affects inducible T regulatory cells in a model of experimental autoimmune encephalomyelitis

Abstract

T regulatory (Treg) cells expressing the transcription factor FoxP3 play a key role in protection against autoimmune disease. GFP-FoxP3 reporter mice have been used widely to study the induction, function and stability of both thymically- and peripherally-induced Treg cells. The N-terminal modification of FoxP3, however, affects its interaction with transcriptional co-factors; this can alter Treg cell development and function in certain self-antigen specific animal models. Interestingly, Treg cell function can be negatively or positively affected, depending on the nature of the model. In this study, we focused on the effect of the GFP-FoxP3 reporter on Treg cell development and function in the Tg4 mouse model. In this model, T cells express a transgenic T cell receptor (TCR) specific for the Myelin Basic Protein (MBP) peptide Ac1-9, making the animals susceptible to experimental autoimmune encephalomyelitis (EAE), a disease akin to multiple sclerosis in humans. Unlike diabetes-susceptible mice, Tg4 FoxP3(gfp) mice did not develop spontaneous autoimmune disease and did not demonstrate augmented susceptibility to induced disease. Concurrently, thymic generation of natural Treg cells was not negatively affected. The induction of FoxP3 expression in naive peripheral T cells was, however, significantly impaired as a result of the transgene. This study shows that the requirements for the interaction of FoxP3 with co-factors, which governs its regulatory ability, differ not only between natural and inducible Treg cells but also between animal models of diseases such as diabetes and EAE.

Figure 1. Young Tg4 FoxP3^gfpmice have unaltered levels of FoxP3 expression.

A. FoxP3 expression by CD4⁺CD8⁻ thymocytes or CD4⁺ cells from the spleen and mesenteric lymph nodes of male Tg4 FoxP3^wt and Tg4 FoxP3^gfp mice aged 5–7 weeks, directly ex vivo. Horizontal bar indicates mean. * p = 0.0329, ** p = 0.0084, n.s. = not significant. 2-tailed, unpaired student's t test, n = 7 each. B. Total number of splenocytes, CD4⁺ T cells and FoxP3⁺CD4⁺ Treg cells in the spleen of male Tg4 mice aged 5–7 weeks. Data displayed as mean, error bar indicates SEM. n = 3 each, * p = 0.0108, n.s. = not significant. 2-tailed, unpaired student's t test. C. Mean fluorescence intensity of FoxP3 staining in Treg cells in the thymus, spleen and mLN of 5–7 week old male Tg4 mice. Data displayed as mean, error bar indicates SEM. n = 3 each, no statistical difference, unpaired, 2-tailed student's t test. D. Frequency of FoxP3 expression on CD4⁺CD8⁻ thymocytes or CD4⁺ cells from the spleen or mLN of Tg4 FoxP3^wt/wt, Tg4 FoxP3^wt/gfp or Tg4 FoxP3^gfp/gfp females aged 5–8 weeks. FoxP3^wt/wt n = 8, FoxP3^wt/gfp n = 12, FoxP3^gfp/gfp n = 4. No statistically significant differences, Tukey's multiple comparison test.

Figure 2. GFP^pos Treg cells display a potentially more suppressive phenotype in Tg4 FoxP3^wt/gfp females.

A. Representative dot plot (splenocytes, left) and distribution graph (right) showing the frequency of GFP expression in CD4⁺FoxP3⁺ cells from the thymus, spleen and mLN of Tg4 FoxP3^wt/gfp females aged 5–8 weeks. Horizontal dotted line represents the predicted frequency. P values show significance of actual vs predicted value. 2-tailed, paired student's t test. n = 10 each. B-H. Representative, smoothed, cytometry plots and distribution graphs showing the frequency of marker expression on GFP^pos and GFP^neg splenic CD4⁺FoxP3⁺ Treg cells in Tg4 FoxP3^wt/gfp females aged 5–10 weeks. P values represent significance determined by 2-tailed, unpaired student's t test. n.s. = not significant. B. Helios, n = 10. C. GITR, n = 10. D. CD103, n = 10. E. PD-1, n = 10. F. CD25, n = 6. G. CD62L, n = 4. H. Ki67, n = 4.

Figure 3. Lower number of peripheral FoxP3⁺ Treg cells in aged Tg4 FoxP3^gfp males.

A. Frequency of FoxP3 expressing CD4⁺ T cells in the spleen of Tg4 FoxP3^wt and Tg4 FoxP3^gfp males aged 16–24 weeks (left) and the frequency of Helios and PD-1 expression gated on CD4⁺FoxP3⁺ splenocytes (middle and right). FoxP3^wt n = 9, FoxP3^gfp n = 6. Data represented as individual scores, with mean±SEM. B. Total number of splenocytes, CD4⁺ T cells or CD4⁺FoxP3⁺ Treg cells in the spleen. FoxP3^wt n = 9, FoxP3^gfp n = 6. Data represented as mean + SEM. C. Mean fluorescence intensity of FoxP3 staining in Treg cells in the spleen. FoxP3^wt, n = 9, FoxP3^gfp n = 6. Data represented as mean+SEM. D. Frequency of Foxp3 expression by CD4⁺ cells in the mLN. FoxP3^wt n = 4, FoxP3^gfp n = 3. Data represented as individual scores, with mean±SEM. E. Percentage of CD44^hi CD4⁺ T cells in the spleen. FoxP3^wt n = 9, FoxP3^gfp n = 6. Data represented as individual scores, with mean±SEM. P values represent significance determined by 2-tailed, unpaired student's t test. n.s. = not significant.

Figure 4. Impaired FoxP3 induction in naive Tg4 FoxP3^gfp CD4⁺ T cells.

A. In vitro FoxP3 induction in male, naive CD4⁺CD62L⁺ T cells by stimulation with peptide and antigen-presenting cells or plate-bound anti-CD3 and anti-CD28 antibody in the presence of IL-2 and TGF-β1 for 7 days. n = 6 each for peptide stimulation, n = 8 each for antibody stimulation. B. Mean fluorescence intensity of anti-FoxP3 antibody staining in FoxP3⁺ iTreg cells. Data represented as mean + SEM. n = 6 each for peptide stimulation, n = 8 each for antibody stimulation. C. In vitro FoxP3 induction in female Tg4 FoxP3^wt/wt and Tg4 FoxP3^wt/gfp naive CD4⁺ T cells. Peptide stimulation; FoxP3^wt/wt n = 5, FoxP3^wt/gfp n = 8. Antibody stimulation; FoxP3^wt/wt n = 10, FoxP3^wt/gfp n = 9. D. Frequency of GFP expression in FoxP3⁺CD4⁺ iTreg cells generated from Tg4 FoxP3^wt/gfp naive T cells using peptide or plate-bound antibody. Peptide stimulation; n = 8, antibody stimulation; n = 10. E. Correlation plot of the frequency of GFP expression in antibody-induced FoxP3⁺ iTreg cells and thymic FoxP3⁺ nTreg cells from the same female Tg4 FoxP3^wt/gfp mice. n = 11. F. Proliferation of CD4⁺ T cells during antibody-mediated iTreg cell generation. Naive female CD4⁺CD62L⁺ Tg4 FoxP3^wt/gfp cells were labeled with 5 µM CPD-eFluor450 prior to 7-day culture with IL-2 and TGF-β₁. Plots gated on FoxP3⁺ Treg cells. Shown are proliferation of GFP^neg and GFP^pos Treg cells from mice with a high (top row) or low (bottom row) frequency of GFP expression. Histogram overlays show proliferation of GFP^neg (black, open line) and GFP^pos (grey, filled line) Treg cells. 2 out of 4 identical experiments (each in triplicate) shown. G. Frequency of Helios and PD-1 expression in GFP^pos and GFP^neg FoxP3⁺CD4⁺ iTreg cells generated from splenic, naive CD4⁺ T cells of Tg4 FoxP3^wt/gfp mice. n = 8. A, B and C; 2-tailed, unpaired student's t test. D. 2-tailed, paired t test on actual measurements versus predicted value of 50%. F and G; 2-tailed, paired student's t test.

Figure 5. Tg4 FoxP3^gfp iTreg cells are suppressive but unstable in vitro.

A. Representative histograms of cell proliferation dye dilution in labeled CD45.1⁺ naive CD4⁺ T cells cultured for 3 days with/without Tg4 FoxP3^wt or Tg4 FoxP3^gfp iTreg cells at a 1∶1 or 2∶1 ratio and stimulated with 10 µg/ml MBP Ac1-9. One representative of three identical experiments in triplicate. B. Proliferation index of naive CD4⁺ cells after 72 h co-culture with/without iTreg cells. Means of 3 identical experiments carried out in triplicate. * p < 0.05, ** p < 0.001, Tukey's multiple comparison test. C. Frequency of FoxP3 expression and proliferation of CD45.2⁺ iTreg cells after 3-day co-culture with naive CD45.1⁺ CD4⁺ T cells. One experiment representative of three carried out in triplicate. Gated on live CD45.2⁺ CD4⁺ T cells. D. Purity of FoxP3⁺ CD4⁺CD25⁺ Tg4 FoxP3^wt/gfp iTreg cells and frequency of GFP expression therein, prior to in vitro co-culture with naive T cells. E. Proliferation and FoxP3 retention of CD45.2⁺ Tg4 FoxP3^wt/gfp iTreg cells as well as frequency of GFP expression among CD45.2⁺ FoxP3⁺ cells after 3-day co-culture with naive CD45.1⁺ CD4⁺ T cells. F. CD25 expression and cell proliferation dye dilution in naive CD45.1⁺ CD4⁺ T cells after 72-h in vitro co-culture with CD45.2⁺ Tg4 FoxP3^wt/wt or Tg4 FoxP3^wt/gfp iTreg cells at different ratios. Proliferation dye dilution displayed as dot plot versus CD25 expression, or off-set histogram overlays. D-F. One experiment representative of 3 identical experiments.

Figure 6. Tg4 iTreg cells are unstable during homeostatic expansion in vivo.

A. Representative histogram overlay of cell proliferation dye dilution of labeled male Tg4 CD45.1⁺ CD4⁺ Tconv cells (left) and proliferation index (mean±SEM), 5 days after co-transfer with/without CD45.2⁺ Tg4 FoxP3^wt or Tg4 FoxP3^gfp iTreg cells into lymphopenic H2URagKO mice. One representative of three identical experiments. B. Representative histogram overlay of cell proliferation dye dilution in labeled female Tg4 CD45.1⁺ CD4⁺ Tconv cells (left) and proliferation index (mean±SEM), 5 days after co-transfer with/without CD45.2⁺ Tg4 FoxP3^wt/gfp iTreg cells into lymphopenic H2URagKO mice. One representative of three identical experiments. C. Purity of FoxP3⁺ CD4⁺CD25⁺ Tg4 FoxP3^wt/gfp iTreg cells and frequency of GFP expression therein, prior to co-transfer with naive T cells. D. Proliferation and FoxP3 retention of CD45.2⁺ Tg4 FoxP3^wt/gfp iTreg cells (left) as well as frequency of GFP expression among CD45.2⁺ FoxP3⁺ cells (right), 5 days after co-transfer with naive CD45.1⁺ CD4⁺ Tconv cells. C and D; One experiment representative of 3 identical experiments.

Figure 7. Tg4 FoxP3gfp mice are not more susceptible to induced disease.

A. Mean EAE disease scores (± SEM) in male and female Tg4 FoxP3 mice, after induction of disease with MBP Ac1-9 in CFA. Male Tg4 FoxP3^wt n = 7, male Tg4 FoxP3^gfp n = 10, female Tg4 FoxP3^wt/wt n = 6, female Tg4 FoxP3^wt/gfp n = 7, female Tg4 FoxP3^gfp/gfp n = 4. B. Mean percentage of initial body weight (on day -1)±SEM, after disease induction. C. disease incidence, mean day of onset (± SD), mean maximum disease grade (± SD) and mortality during the first 21 days following disease induction.