Defective CD8+ T Cell Activation and Cytolytic Function in the Absence of LFA-1 Cannot Be Restored by Increased TCR Signaling

Signaling through the TCR as well as engagement of costimulatory molecules are required for efficient T cell activation and progression into differentiated effector cells. The β2 integrin LFA-1 (CD11a/CD18) has been implicated in TCR costimulation as well as in cell-cell adhesion function, but its exact role is still ambiguous. The present study focuses on the requirement for LFA-1 in CD8+ T cell activation and effector function using LFA-1-deficient cells expressing the 2C transgenic TCR as a model system. The lack of LFA-1 expression in 2C T cells resulted in severely diminished proliferative response toward allogeneic BALB/c splenocytes. Increase in TCR signaling alone by pulsing stimulators with high affinity peptides, p2Ca or QL9, had minimal effects in restoring proliferation. Addition of exogenous IL-2, however, enhanced the effect of peptide pulsing on proliferation of LFA-1-deficient 2C T cells. LFA-1-deficient 2C CTLs generated from alloantigen stimulation exhibited a defective cytotoxic activity when tested on a variety of target cells. Cytolysis could be improved, but not fully rectified by peptide pulsing of target cells. Thus, in the 2C TCR model, LFA-1 has a requisite role for optimal CD8+ T cell activation and effector function, which cannot be overcome by increasing peptide/MHC density on either the APCs or target cells, respectively.

The Dependence for Leukocyte Function-Associated Antigen-1/ICAM-1 Interactions in T Cell Activation Cannot Be Overcome by Expression of High Density TCR Ligand

The leukocyte-specific integrin, LFA-1, can enhance T cell activation. However, it is unclear whether the binding of LFA-1 to its ligand, ICAM-1, functions through intercellular adhesion alone, resulting in an augmentation of the TCR signal, or involves an additional LFA-1-mediated cellular signal transduction pathway. We have previously shown that naive CD4+ lymph node T cells, isolated from DO11.10 TCR transgenic mice, are activated by increasing doses of exogenous OVA peptide presented by transfectants expressing both class II and ICAM-1, but not by cells expressing class II alone. To determine whether LFA-1/ICAM-1 interactions were simply enhancing the presentation of low concentrations of specific MHC/peptide complexes generated from exogenously added peptide, we transfected cells with class II that is covalently coupled to peptide, alone or in combination with ICAM-1. These cells express 100-fold more specific class II/peptide complexes than can be loaded onto class II-positive cells at maximum concentrations of exogenous peptide. Despite this high density of TCR ligand, activation of naive CD4+ T cells still requires the coexpression of ICAM-1. LFA-1/ICAM-1 interactions are not required for effective conjugate formation and TCR engagement because presentation of class II/peptide complexes in the absence of ICAM-1 does induce up-regulation of CD25 and CD69. Thus, high numbers of engaged TCR cannot compensate for the lack of LFA-1/ICAM-1 interactions in the activation of naive CD4+ T cells.

Functional Consequences of Costimulation by ICAM-1 on IL-2 Gene Expression and T Cell Activation

LFA-1 is a well-recognized adhesion molecule, but its role in providing costimulatory signals to T cells has remained controversial. We have compared the ability of class II-positive transfectants that do and do not coexpress ICAM-1 (ProAd and ProAd-ICAM) to activate Ag-specific Th1 clones and naive CD4-positive T cells isolated from TCR transgenic mice. Ag presentation by ProAd to Th1 clones can induce calcium-dependent signaling events after engagement of the TCR, as evidenced by the nuclear localization of the transcription factors NF-AT and NF-κB. Nevertheless, coexpression of ICAM-1 or B7-1 on ProAd is required to induce detectable levels of IL-2 gene expression in either Th1 clones or naive T cells. In Th1 clones, activation by ProAd-ICAM induces very transient IL-2 mRNA expression that does not result in detectable IL-2 secretion or T cell proliferation. In naive T cells, the duration of IL-2 mRNA expression is longer, allowing for a transient burst of IL-2 protein that is sufficient to drive the cells into the cell cycle. In spite of this initial response, Ag presentation by ProAd-ICAM is a tolerogenic signal to naive T cells, and responding T cells undergo apoptosis 4 to 5 days poststimulation. These data suggest that engagement of LFA-1 can provide sufficient costimulatory signals to induce T cell activation and IL-2 gene expression, but cannot protect against anergy induction or provide for T cell survival.

Intercellular Adhesion Molecule-1 and Leukocyte Function-Associated Antigen-3 Provide Costimulation for Superantigen-Induced T Lymphocyte Proliferation in the Absence of a Specific Presenting Molecule

Bacterial superantigens can bind TCR in the absence of MHC class II molecules and activate T lymphocytes when cocultured with certain class II-deficient accessory cells. It has not been determined, however, whether these accessory cells provide direct costimulation to the T cell or serve to present superantigens via a nonconventional ligand. We have identified a human adenocarcinoma cell line, SW480, that assists in the activation of human T cells by the staphylococcal enterotoxins B (SEB), C1 (SEC1), and D (SED), but not SEA, SEC2, SEC3, or SEE. SW480 cells did not express class II molecules, and anti-class II mAbs did not inhibit T cell proliferation, supporting the hypothesis that class II is not absolutely required for enterotoxin-mediated T cell activation. The TCR Vβ profile of T cells stimulated by SEB plus SW480 cells was similar to that of T cells stimulated by SEB plus class II+ APC, indicating that TCR-SEB interactions were preserved in the absence of class II molecules. Binding studies failed to detect specific association of SEB with SW480 cells, suggesting that SW480 cells do not express receptors for enterotoxin. SEB coupled to beads, however, stimulated T cell proliferation, but only in the presence of SW480 cells. SW480 cells express both ICAM-1 and LFA-3 molecules, and the addition of Abs to these receptors inhibited T cell proliferation. These findings support a model in which certain enterotoxins engage the TCR independent of MHC class II or other specific presenting molecules and induce T cell proliferation with signals provided by nonconventional accessory cells.