Recent studies have suggested a role for MHC class Ib molecules in providing signals for memory T cell differentiation during the early phases of acute infection. To test this hypothesis, we assessed the development of effector and memory CD8 T cells in transgenic mice expressing a single chain H-2Dd/β2-microglobulin (β2M) fusion protein on a β2M-deficient background. These mice thus express a single MHC class Ia in the absence of all other β2M-dependent class Ia and Ib molecules. Following infection with a recombinant vaccinia virus expressing a known Dd-restricted epitope from HIV-1 gp160, the development of effector and memory cells CD8 T cells was comparable to control mice. Furthermore, these memory cells responded rapidly and robustly to antigenic restimulation. Therefore, we conclude that full CD8 memory differentiation requires only a single MHC class Ia chain, ruling out a requirement for MHC class Ib molecules in this process.
Following acute infection, naive Ag-specific CD8 T cells undergo a remarkable period of activation and proliferation, with as much as a 10,000- to 50,000-fold expansion in numbers (1). Following the peak of the primary response, 90–95% of the activated effector cells die, leaving behind a population of stable, long-lived memory cells (2, 3). These cells are characterized by their ability to confer long-term protection against reinfection due to their increased precursor frequency, faster response time, and ability to persist virtually indefinitely in the absence of Ag (4).
The early events following acute infection that direct some CD8 T cells to become end-stage effector cells while directing others to become long-lived memory cells remain largely unexplained. A more thorough understanding of these mechanisms will not only shed light on the fundamental immunological question of how long-term protective immunity is established, but also provide insight into the design of safer vaccines that can more effectively generate memory cells in vivo. One recent study identified IL-7Rα as a marker for memory cell precursors (5). A small portion (∼10%) of effector CD8 T cells at the peak of the immune response were found to express IL-7Rα, and these cells preferentially survived to become long-lived, functionally protective memory cells. Expression of IL-7Rα was also shown to be functionally important for the survival of these memory precursors.
Recent work has also implicated a role for MHC class Ib molecules in the development of T cell memory (6, 7). One study correlated the transient expression of the CD8αα homodimer on Ag-specific CD8 T cells in the spleen with that of IL-7Rα, indicating that early CD8αα expression may also be a marker for T cell memory development. Transfer of the CD8αα-expressing effector cell subset showed their preferential survival and development into memory cells. Furthermore, memory development was impaired in mice that lacked the ability to up-regulate CD8αα on CD8αβ T cells during acute infection, leading the authors to suggest that CD8αα binding to its ligand, the MHC class Ib molecule TL, is a necessary step in the differentiation of CD8 memory precursors (7). Because TL binds CD8αα homodimers with much greater affinity than CD8αβ heterodimers, and without the need for peptide specificity (8), it has been proposed that this interaction can modify lck signaling by redirecting CD8αα away from the TCR activation complex (9).
We sought to test directly whether nonclassical MHC molecules played a role in memory T cell development by studying immune responses to acute infection in transgenic mice that express a single chain (Sc) 3 H-2Dd under the control of a truncated MHC class I promoter in which the N terminus of the H chain is covalently linked to the C terminus of β2M (10, 11). Because the transgenic mice are on a β2-microglobulin (β2M)-deficient background (hereafter referred to as ScDdβ2M−/−), no other β2M-dependent MHC class Ia or class Ib molecules, such as TL, is expressed. Following infection with a recombinant vaccinia virus (rVV; Ref. 3) (vPE16; Ref.12) expressing an immunodominant Dd-restricted epitope from HIV-1 gp160 (P18-I10) (13), we observe normal development of both effector and memory CD8 populations. Both effector and memory CD8 T cells in ScDdβ2M−/− mice express levels of effector cytokines and IL-7Rα that are comparable to control mice. Furthermore, memory T cells derived in the transgenic mice respond robustly following restimulation, leading us to conclude that a single MHC class Ia is sufficient for the development of CD8 T cell memory, and that MHC class Ib molecules are not required in this process.
Materials and Methods
Six- to 8-wk-old B10.D2 and C57BL/6 mice were purchased from The Jackson Laboratory. C57BL/6NCr tScβ2mDd transgenic mice expressing a Sc H-2Dd/β2M fusion construct were kindly provided by D. Margulies (Molecular Biology Section, National Institute of Allergy and Infectious Diseases, Bethesda, MD) and maintained on a B6.129-B2mtm1Jae (β2M−/−; Taconic Farms) background at our breeding facilities. These mice will hereafter be referred to as ScDd β2M−/−. All mouse experiments were performed with the approval of the Institutional Animal Care and Use Committee at the University of Washington.
Mice 8–12 wk of age were infected with 2 × 106 PFU i.p. of a rVV expressing HIV-1 envelope glycoprotein gp160 (vPE16; kindly provided by B. Moss, Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, Bethesda, MD) (12). The P18-I10 peptide (RGPGRAFVTI) of this protein is H-2Dd-restricted (13).
Intracellular cytokine staining and peptides
Intracellular cytokine staining was performed essentially as described (14). Splenocytes, mesenteric lymph node cells, and liver lymphocytes were resuspended in RP-10 (RPMI 1640 supplemented with 10% FCS, 2 mM l-glutamine, 10 mM HEPES, 0.5 μM 2-ME, 100 U/ml penicillin, and 100 μg/ml streptomycin). A total of 2 × 106 cells/well were plated in 96-well plates in the presence of 1 μl/ml Brefeldin A (GolgiPlug, Cytofix/Cytoperm kit; BD Pharmingen) with or without the appropriate peptide for 4–5 h at 37°C. P18IIIB-I10 peptide (RGPGRAFVTI) was added at a concentration of 0.1 μg/ml. Following the incubation, cells were stained with CD8-FITC and permeabilized and stained for intracellular cytokine expression using reagents provided in the Cytofix/Cytoperm kit according to the manufacturer’s instructions (BD Pharmingen). The Abs used were IFN-γ-allophycocyanin and IL-2-PE or IL-7Rα-PE (BD Pharmingen), and cells were subsequently analyzed by flow cytometry.
In vitro restimulations
Splenocytes were incubated in 5 μM CFSE in RPMI 1640 (Molecular Probes). After 10 min the staining was halted by the addition of cold RPMI 1640. Cells were washed, resuspended in RP-10 at 2 × 106 cells/well in a 96-well plate, and incubated with 100 nM, 10 nM, or 1 nM P18-I10 peptide. Cells were harvested 3 or 5 days later and analyzed by flow cytometry for CFSE dilution. In duplicate wells, Brefeldin A (GolgiPlug, Cytofix/Cytoperm kit; BD Pharmingen) was added for the final 3 h of stimulation, and cells were subsequently stained and analyzed for intracellular IFN-γ expression.
Ab staining and FACS analysis
Cells were stained with the following Abs: CD8-FITC, CD8-PE, IL-2-PE, IL-7Rα-PE, CD25-allophycocyanin, IFN-γ-allophycocyanin, H-2Dd-FITC, H-2Kb-PE, CD4-PerCP, CD8-allophycocyanin (BD Pharmingen). Cells were analyzed using a FACSCalibur (BD Biosciences) and FlowJo software (Tree Star).
Results and Discussion
ScDdβ2M−/− mice, which express a Dd/β2M fusion protein on a β2M-deficient background, have been previously characterized as generating normal numbers of functionally responsive CD8 T cells in the periphery (11). To confirm this, we stained these mice for expression of H2-Kb, H-2Dd, CD4, and CD8 in the spleen and compared them to wild-type mice. As expected, we found that only H-2Dd is detectably expressed in the Dd-transgenic mice, and they generate close to normal numbers of CD8 T cells in the periphery (Fig. 1).
To test the ability of these mice to respond to acute infection, we infected them with 2 × 106 PFU i.p. with a rVV expressing the HIV-1 envelope glycoprotein gp160 (vPE16). Infection with this virus has previously been shown to stimulate a response to the H-2Dd-restricted P18-I10 peptide of the glycoprotein (11). In accordance with this, we observed a robust response to this epitope in the spleen by day 8 postinfection (p.i.) in both wild-type B10.D2 and ScDdβ2M−/− mice, as observed by intracellular IFN-γ staining following ex vivo restimulation (Fig. 2, A and C). In fact, the response to this epitope in the transgenic mice was three to five times larger than that seen in B10.D2 controls. This observation may be due either to a higher precursor frequency of Ag-specific cells, as the entire CD8 repertoire is H-2Dd-restricted, or to a lack of competition with epitopes restricted to other class I MHC. Similar results were obtained in the lymph nodes and liver (data not shown).
At 90 days p.i. the frequency of P18-I10-specific memory CD8 T cells was assessed in the spleen, lymph nodes, and liver. Memory cells were readily detectable in the spleen in both the B10.D2 controls and the ScDdβ2M−/− mice. The numbers of Ag-specific cells corresponded with the extent of the initial primary response, with the B10.D2 memory cells representing 5.2% and the ScDdβ2M−/− memory cells representing 12.8% of the initial response (Fig. 2, B and C). Once again, similar results were obtained upon analysis of lymph nodes cells and liver lymphocytes in ScDdβ2M−/− mice (data not shown).
Furthermore, the memory cells present in the transgenic mice are similar in phenotype and functional capacity to control mice, as measured by levels of IFN-γ and IL-2 production, and expression of IL-7Rα. In both control B10.D2 and ScDdβ2M−/− mice, 80–90% of the P18-I10-specific CD8 memory T cells also expressed IL7Rα, and ∼50% of the memory cells coproduced IL-2 along with IFN-γ (Fig. 3). Furthermore, the levels of IFN-γ produced by the memory cells in each group of mice was equivalent, as measured by mean fluorescence intensity (MFI) (data not shown).
We sought to determine whether these cells were equally capable of responding to restimulation. Splenocytes from ScDdβ2M−/− or B10.D2 memory mice were stained with CFSE and cultured with or without 10 nM P18-I10 peptide for either 3 or 5 days in vitro. Based on the number of cells that diluted their CFSE, as well as the number of divisions, memory cells in both groups of mice proliferated comparably in response to restimulation, resulting in a 3- to 5-fold expansion by day 3 and a 20- to 30-fold expansion by day 5 for both groups (Fig. 4, A and B). Furthermore, memory cells from both groups of mice generated comparable and robust IFN-γ responses (Fig. 4 C). Responses for each group were comparable over a range of peptide concentrations from 1 nM to 1 μM (data not shown).
These experiments demonstrate that a single MHC class Ia, in the absence of any other β2M-dependent MHC class Ia or Ib, can dictate the full differentiation of CD8 memory cells. Effector and memory cells generated in ScDdβ2M−/− mice are phenotypically and functionally similar to those seen in B10.D2 control mice. Furthermore, the resulting memory cells retain the capacity to respond rapidly and robustly to restimulation. Therefore, it is clear that TL, along with other β2M-dependent class Ib molecules, does not play an obligatory role in the differentiation of CD8 memory following acute infection. Nevertheless, the possibility exists that individual class Ib molecules may play positive or negative roles in providing signals for T cell differentiation that are not readily apparent in our mice which lack all β2M-dependent class Ib molecules. It also remains possible that CD8αα has an alternative ligand, and that binding to this ligand relays an important memory differentiation signal.
The authors have no financial conflict of interest.
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
This work was supported by the Howard Hughes Medical Institute and National Institutes of Health Grant AI19335 (to M.J.B.) and by Ruth L. Kirchstein National Research Service Award Fellowship AI056809 (to M.A.W.).
Abbreviations used in this paper: Sc, single chain; β2M, β2-microglobulin; rVV, recombinant vaccinia virus; MFI, mean fluorescence intensity.