Diversity of the CD4 T Cell Alloresponse: The Short and the Long of It

Abstract

MHC alloantigen is recognized by two pathways: "directly," intact on donor cells, or "indirectly," as self-restricted allopeptide. The duration of each pathway, and its relative contribution to allograft vasculopathy, remain unclear. Using a murine model of chronic allograft rejection, we report that direct-pathway CD4 T cell alloresponses, as well as indirect-pathway responses against MHC class II alloantigen, are curtailed by rapid elimination of donor hematopoietic antigen-presenting cells. In contrast, persistent presentation of epitope resulted in continual division and less-profound contraction of the class I allopeptide-specific CD4 T cell population, with approximately 10,000-fold more cells persisting than following acute allograft rejection. This expanded population nevertheless displayed sub-optimal anamnestic responses and was unable to provide co-stimulation-independent help for generating alloantibody. Indirect-pathway CD4 T cell responses are heterogeneous. Appreciation that responses against particular alloantigens dominate at late time points will likely inform development of strategies aimed at improving transplant outcomes.

Graphical abstract

Figure 1

Characterization of MHC-Mismatched Murine Model of Chronic Heart Allograft Vasculopathy (A) B6.Kd, bm12, and ABOIE mice were inter-crossed to create a “bm12.kd.IE” donor strain that differed from the C57BL/6 recipient strain at the classical MHC class I H-2K and MHC class II I-A and I-E loci, enabling direct-pathway CD4 T cell alloimmunity to be assessed by adoptive transfer of CFSE-labeled, TCR-transgenic ABM CD4 T cells. (B) I-A^b-restricted indirect pathway CD4 T cell responses against MHC class I H-2K^d, MHC class II IE, and minor H-Y dby antigen could be similarly assessed by transfer of TCR-transgenic TCR75, Tea, and Marilyn CD4 T cells, respectively. (C and D) Whereas syngeneic C57BL/6 heart grafts survive indefinitely, bm12.kd.IE heart allografts are rejected slowly by C57BL/6 recipients (C) with development of progressive allograft vasculopathy (D): representative photomicrographs of elastin van-Gieson-stained paraffin sections depicting typical fibroproliferative arterial intimal thickening observed in rejecting heart allografts (scale bars, 100 μm). (E–H) Rejection is associated with development of germinal centers (E), class-switched anti-nuclear autoantibody (F), and anti H-2K^d (G) and anti-I-E (H) alloantibody responses. ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 (Kaplan-Meier log rank analysis in C, Mann-Whitney test in D and E, and two-way ANOVA in F–H). Data represent mean and SEM of n = 7 mice per group.

Figure 2

Heterogeneity of Indirect-Pathway CD4 T Cell Responses (A) Following transplantation of female C57BL/6 recipients with a male bm12.Kd.IE heart allograft, indirect pathway CD4 T cell responses against mismatched MHC class I H-2K^d, MHC class II I-E, and minor H-Y antigen were assessed by quantifying division (expressed as percentage of cells undergoing at least one division) of CFSE-labeled TCR75, Tea, and Marilyn CD4 T cells, respectively, transferred at either transplantation or 5 weeks later. (B) Whereas CFSE-labeled TEa CD4 T cells proliferate robustly when transferred at transplantation of a C57BL/6 recipient with a bm12.Kd.IE heart allograft, no division is observed when TEa CD4 T cells are transferred 1 week later. (C) Division profiles of TEa CD4 T cells transferred to C57BL/6 recipients at transplantation with bm12.Kd.IE heart allografts, with the donors lethally irradiated or either the donor or recipient mice depleted of B cells and/or DCs. Division profiles following transfer of memory TEa CD4 T cells to C57BL/6 recipients of a bm12.Kd.IE heart allograft are shown. Profiles of transferred naive TEa CD4 T cells are replicated from (A) for comparison. (D) Also shown is division of naive TEa CD4 T cells transferred 5 weeks after transplantation to a C57BL/6 recipient that received bm12.Kd.IE BMDCs the day before TEa transfer. Statistical comparison is to untreated group. (E) YAe clonotypic antibody was used to identify I-A^b-restricted I-E peptide epitope on purified DCs. (Left panel) Overlay histogram of YAe staining of splenic DCs from naive C57BL/6 mice (dotted), naive BALB/c x C57BL/6 F1 (CB6F1) mice (dashed), and DCs purified from C57BL/6 recipients 5 days after bm12.Kd.IE heart transplantation (solid) is shown. (Right panel) Comparative YAe staining of splenic DCs from C57BL/6 recipients 1 week after transplantation with DC- and B cell-depleted bm12.Kd.IE heart allografts; C57BL/6 recipients 2 weeks after bm12.Kd.IE heart transplantation; and H-2^bRag2^−/− recipients 5 weeks after bm12.Kd.IE heart transplantation is shown (DCs purified from C57BL/6 recipients 5 days after bm12.Kd.IE heart transplantation are overlaid for comparison—shaded). ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 (Mann-Whitney test in A, C, and D). Data are representative of two independent experiments resulting in n = 5 in each group (A, C, and D; mean and SEM of n = 5 mice per group).

Figure 3

Endothelial Expression of MHC Class II Antigen (A) C57BL/6 (blue) and bm12.kd.IE (red) endothelial cells were cultured from neonatal cardiomyocytes, with representative flow cytometric overlay histograms confirming (top panels) upregulated expression of CD105 and isolectin B4. (B) Cultured C57BL/6 endothelial cells expressed exclusively MHC class II I-Ab, whereas cultured bm12.kd.IE endothelial cells expressed exclusively MHC class II I-E antigens (lower panels: shaded histograms isotype control antibody). Immunofluorescent staining with anti-CD31 (endothelial) antibody and with either anti-I-E or anti-I-Ab antibodies of cryostat sections of bm12.Kd.IE heart allografts explanted 1 week after transplantation into C57BL/6 recipients is shown (scale bars, 20 μm). Data are representative of one experiment (A) and three independent experiments (B), with n = 5 mice per group.

Figure 4

Persistent Indirect-Pathway CD4 T Cell Activation from Graft Parenchymal Expression of MHC Class I (A) Indirect-pathway CD4 T cell responses against MHC class I alloantigen were assessed, as detailed in Figure 2, by quantifying division of CFSE-labeled TCR75 T cells, adoptively transferred into C57BL/6 recipients at transplantation with a bm12.Kd.IE allograft; neither donor lethal irradiation nor donor DC depletion inhibited TCR75 CD4 T cell division. (B) Bone marrow chimeric recipients were created as depicted, by transferring bm12.Kd.IE bone marrow into lethally irradiated bm12.IE mice: lower panels, representative flow cytometry plots demonstrating H-2K^d expression in chimeric mice on hematopoietic cells (splenic B cells). (C) Heart allografts from bm12.Kd.IE, bm12.IE, or bm12.Kd.IE→bm12.IE bone marrow chimeric mice were transplanted into C57BL/6 recipients and division of TCR75 CD4 T cells (transferred at the time of, or 5 weeks after, transplantation) quantified. (D) BALB/c heart allografts were transplanted into C57BL/6 recipients that were either left untreated or administered anti-CD154 at transplantation and division of TCR75 CD4 T cells (left panel) or TEa CD4 T cells (right), transferred at the times indicated, quantified. ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 (Mann-Whitney test in A, C, and D). Data are representative of two independent experiments resulting in n = 5 in each group (A, C, and D; mean and SEM of n = 5 mice per group).

Figure 5

Direct-Pathway CD4 T Cell Responses Are Short Lived and Curtailed by Innate and Adaptive Alloimmunity (A) Direct-pathway CD4 T cell responses were assessed in C57BL/6 recipients of bm12.Kd.IE cardiac allografts by quantifying division of I-A^bm12-reactive ABM CD4 T cells, adoptively transferred at transplantation or 5 weeks later. Division of ABM CD4 T cells following transfer into naive C57BL/6 recipients is shown for comparison. (B) Direct-pathway responses were prolonged in Rag2^−/− recipients of an unmodified bm12.Kd.IE heart allograft. (C) Direct-pathway CD4 T cell alloresponses were also assessed in bm12 recipients of a BALB/c × C57BL/6 F1 (CB6F1) heart allograft by adoptive transfer of TCR75 CD4 T cells. In this strain combination, TCR75 CD4 T cells do not recognize recipient I-A^bm12-restricted H-2K^d allopeptide but respond to H-2K^d-peptide presented by I-A^b on donor cells. (D) TCR75 CD4 T cell direct-pathway responses are short lived and significantly downregulated by lethal irradiation of the donor or depletion of either donor B cells or DCs. (E) Syngeneic C57BL/6 or allogeneic CB6F1 CD4 T cells were injected into either T- and B-cell-deficient C57BL/6 Rag2^−/− mice (left panel) or Rag2IL2rg mice that additionally lack NK cells (right panel) and the presence of circulating injected cells (expressed as percentage of PBMCs) assessed by flow cytometry. (F) Direct-pathway responses were assessed as in (B) by quantifying division of TCR75 CD4 T cells that were adoptively transferred at the time of, or 1 week after (week 2), transplantation of CB6F1 heart grafts into wild-type, NK cell-depleted, or T-cell-deficient bm12 recipients. ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 (Mann-Whitney test in A, B, D, and F). Data are representative of two independent experiments resulting in n = 5 in each group (A, B, D, and F; mean and SEM of n = 5 mice per group) and one experiment with n = 3 in each group (E).

Figure 6

Late Allopeptide Presentation Shapes the Dynamics of the Endogenous CD4 T Cell Alloresponse (A) The endogenous H-2K^d_54-68 allopeptide-specific splenic CD4 T cell subset in C57BL/6 recipients of bm12.Kd.IE or BALB/c heart allografts was identified by labeling with H-2K^d_54–68/I-A^b tetramers: representative flow cytometry plots of binding of splenocytes purified from bm12.Kd.IE heart grafted recipients to (left panels) H-2K^d_54–68/I-A^b tetramer (experimental) or to control I-A^b tetramer bound with human CLIP_87–101 (class-II-associated invariant chain peptide) and quantification of absolute numbers of tetramer-positive cells present in C57BL/6 recipients at weekly intervals after transplant of bm12.Kd.IE or BALB/c heart allografts (right panel). (B) Prevention of acute rejection by anti-CD154 mAb treatment of C57BL/6 recipients of a BALB/c heart allograft augments numbers of H-2K^d-allopeptide-specific CD4 T cells 7 weeks after transplant. (C) H-2K^d-allopeptide-specific CD4 T cells in cell cycle, identified by Ki-67 labeling 5 weeks after transplantation and expressed as percentage of total tetramer-positive cells. (D) Within the antigen-experienced CD44^hi tetramer-positive CD4 T cell population (top left panel), CCR7^hiCD62L^hi central (bottom left) and CCR7^loCD62L^lo effector (bottom right) memory T cells were quantified (gating strategy shown on representative flow cytometry plot) 7 weeks after transplantation of BALB/c or bm12.Kd.IE heart allografts into C57BL/6 recipients. (E) Anti H-2K^d IgG alloantibody responses to a B6.K^d heart allograft in Tcrbd^−/− recipients, treated with anti-CD154 mAb and reconstituted with splenic CD4 T cells purified from C57BL/6 recipients 5 weeks after transplantation with BALB/c or bm12.Kd.IE heart allografts. ^∗p < 0.05, ^∗∗p < 0.01, and ^∗∗∗p < 0.001 (two-way ANOVA in A and E and Mann-Whitney test in B–D). Data are representative of one experiment with n = 4 in each group (A–C, and E; mean and SEM of n = 4 mice per group) and of two independent experiments with a total of n = 5 in each group (D).