Characterization of the major histocompatibility complex class II binding site on LAG-3 protein

T Lymphocytes infiltrating various tumour types express the MHC class II ligand lymphocyte activation gene-3 (LAG-3): role of LAG-3/MHC class II interactions in cell-cell contacts

The product of the Lymphocyte Activation Gene-3 (LAG-3, CD223) is a high affinity MHC class II ligand expressed by activated CD4(+) and CD8(+) T cells, which can associate with the T cell receptor (TCR) and downregulate TCR signalling in vitro. We have also reported that a soluble mLAG-3Ig fusion protein works as a vaccine adjuvant in vivo in mice, enhancing Th1 and CD8 T cell responses. Here, we report that LAG-3 expression was found, using fluorescent activated cell sorting (FACS) analysis, on 11-48% of human tumour-infiltrating lymphocytes (TILs) isolated from eight freshly dissociated renal cell carcinomas (RCCs), and was restricted mostly to CD8(+) cells. Immunohistochemical analysis confirmed LAG-3 expression by TILs in 9/11 RCCs, as well as in tumours of different origins, such as melanomas (3/5) and lymphomas (7/7). Since not only antigen presenting cells (APCs), but also TILs themselves strongly express major histocompatibility complex (MHC) class II, we firstly investigated whether LAG-3/MHC class II T-T cell contacts might influence tumour cell recognition. However, cytotoxicity inhibition was not observed in two RCC-specific CD8(+) T cell clones in the presence of the LAG-3-specific MAb, and there was also no observed difference in the recognition of LAG-3-transfected or wild-type RCC by these cytotoxic T lymphocytes (CTLs). In contrast, MHC class II engagement by LAG-3Ig was found to enhance the capacity of immature dendritic cells to stimulate naive T cell proliferation and IL-12-dependent IFN-gamma production by T cells in vitro. These results therefore provide support for a role for TIL-expressed LAG-3 in the engagement of class II molecules on APCs, thereby contributing to APC activation and Th1/Tc1 commitment, without downregulating cytotoxicity.

Human natural killer cell receptors and signal transduction

Natural killer (NK) cells express numerous receptors, which continually engage with ligands on cell surfaces. Until 1995, only a handful of these receptors were characterized and the molecular basis of NK cell activation was obscure. Recently, considerable advances have been made in characterizing the receptor repertoire on human NK cells. Both activating and inhibitory receptors can transduce positive or negative signals to regulate NK cell cytotoxicity and cytokine release responses. The inhibitory receptors normally predominate in this balance of signals. Certain tumor cells and virally infected cells that lack major histocompatibility complex (MHC) class I molecules, however, can rapidly trigger NK cell activation. The basis of this activation is the loss of negative signals that are normally transmitted by MHC class I-binding inhibitory receptors, and the corresponding domination of activating receptor signals. While ligand specificity for a number of the recently described receptors is still a mystery, their signal transduction properties have begun to be defined. The dynamic crosstalk between these receptors ultimately governs the NK cell activation state. Although the complexities of NK cell signalling are only marginally understood, several overall themes have been defined by characterizing the roles of distinct pathways during NK cell responses.

A soluble lymphocyte activation gene-3 molecule used as a vaccine adjuvant elicits greater humoral and cellular immune responses to both particulate and soluble antigens

The lymphocyte activation gene-3 (LAG-3) product is a MHC class II ligand that has been used in vivo to stimulate MHC class II+ APCs to increase tumor-specific immune responses. We investigated whether LAG-3 could also play an adjuvant role in vivo for the induction of humoral and CD4 or CD8 cell-mediated immune responses when immunizing mice with a particulate (hepatitis B surface Ag) or soluble (OVA) Ag. In both cases, coadministration of 1 microg of a soluble fusion protein between murine LAG-3 and the Fc fraction of a murine IgG2a mAb (mLAG-3Ig) as a vaccine adjuvant induced or increased CTL responses to the corresponding MHC class I-restricted peptide. In addition, splenocytes of mice vaccinated with either the particulate or soluble Ag plus mLAG-3Ig exhibited a significantly greater proliferative response than did splenocytes of mice immunized with Ag and a control Ig molecule. Similarly, these splenocytes had a greater Th1- but not Th2-type cytokine response. Finally, mice immunized with Ag plus mLAG-3Ig produced higher titers of Abs than mice immunized with Ag and a control Ig molecule. Thus, these data provide evidence of a novel means of improving the immunogenicity of subunit vaccines.

Lymphocyte activation gene-3 induces tumor regression and antitumor immune responses

The lymphocyte activation gene-3 (LAG-3) product is an MHC class II ligand related to CD4. We investigated whether LAG-3 could be used in vivo to stimulate MHC class II(+) antigen-presenting cells (APC), such as resident macrophages or dendritic cells known to play a crucial role in processing and presenting of antigens to the immune system. We first introduced human (h) LAG-3 or mouse LAG-3 into three types of tumor cells (MCA 205, TS / A and RENCA) to evaluate its capacity to stimulate a tumor-specific immune response in vivo. In contrast to the progressive growth of wild-type cells in syngeneic mice, LAG-3-transfected tumors completely regressed or their growth was markedly reduced. Mice were significantly to completely protected against a rechallenge with parental tumor cells. Protection induced by hLAG-3(+) tumor cells involved recruitment of a CD8(+) T cell response since nu / nu mice and CD8-depleted mice did not reject tumors, and a systemic tumor-specific CTL activity was induced. Co-administration of soluble LAG-3 with wild-type tumor cells also markedly reduced primary tumor growth. Interestingly, immunization with LAG-3(+) tumor cells or co-administration of soluble LAG-3 with irradiated wild-type tumor cells reduced the growth of pre-established tumors. We therefore suggest that LAG-3 could be used as a vaccine adjuvant for its ability to trigger APC via MHC class II molecules.

The MHC class II ligand lymphocyte activation gene-3 is co-distributed with CD8 and CD3-TCR molecules after their engagement by mAb or peptide-MHC class I complexes

Previous studies indicated that signaling through lymphocyte activation gene-3 (LAG-3), a MHC class II ligand, induced by multivalent anti-receptor antibodies led to unresponsiveness to TCR stimulation. Here, lateral distribution of the LAG-3 molecules and its topological relationship (mutual proximity) to the TCR, CD8, CD4, and MHC class I and II molecules were studied in the plasma membrane of activated human T cells in co-capping experiments and conventional fluorescence microscopy. Following TCR engagement by either TCR-specific mAb or MHC-peptide complex recognition in T-B cell conjugates, LAG-3 was found to be specifically associated with the CD3-TCR complex. Similarly, following CD8 engagement LAG-3 and CD8 were co-distributed on the cell surface while only a low percentage of CD4-capped cells displayed LAG-3 co-caps. In addition, LAG-3 was found to be associated with MHC class II (i.e. DR, DP and DQ) and partially with MHC class I molecules. The supramolecular assemblies described here between LAG-3, CD3, CD8 and MHC class II molecules may result from an organization in raft microdomains, a phenomenon known to regulate early events of T cell activation.

CD3/TCR Complex-Associated Lymphocyte Activation Gene-3 Molecules Inhibit CD3/TCR Signaling

The lymphocyte activation gene-3 (LAG-3) molecule is a T cell activation Ag closely related to CD4 at the gene and protein levels. We investigated whether LAG-3 itself may down-regulate the immune response by interfering with TCR signaling. The binding of Ab to the LAG-3 molecule followed by cross-linking (XL) inhibits cell proliferation and cytokine secretion in response to CD3XL on activated T cells. LAG-3XL-induced down-regulation is associated with functional unresponsiveness, as well as with high CD25 expression levels and reversion by exogenous IL-2. It is also associated with a down-modulation of CD3/TCR complex expression. At the biochemical level, LAG-3XL inhibits calcium response to CD3 stimulation. This inhibition is observed with different LAG-3- and CD3-specific mAbs on condition that the two receptors are cross-linked together. Finally, the capping of CD3 was shown to induce cocapping of LAG-3 molecules. Together, these results show that CD3/TCR complex-associated LAG-3 molecules can play an active role in negatively regulating the CD3/TCR activation pathway. They ultimately suggest that LAG-3 is an inhibitory receptor in activated T lymphocytes.

Lymphocyte Activation Gene-3 (LAG-3) Expression and IFN-γ Production Are Variably Coregulated in Different Human T Lymphocyte Subpopulations

We evaluated the relationship between cytokine profile and the expression of the lymphocyte activation gene-3 (LAG-3) in both T cell clones and polyclonal T cell lines; LAG-3 is a CD4-like protein whose expression is reportedly restricted to Th1/0 cells and dependent upon IFN-γ. We found that, while LAG-3 was expressed only by CD4+ T cell clones producing IFN-γ, most CD8+ clones producing IL-4 but not IFN-γ (i.e., with a T cytotoxic-2-like profile) were LAG-3+. The intensity of LAG-3 expression by CD8+ clones correlated with the amount of released IFN-γ, suggesting that this cytokine is not required for expression but rather for the up-regulation of LAG-3. Flow cytometric analyses of polyclonal T cell lines confirmed that LAG-3 could be expressed by both CD4+ and CD8+ cells that did not contain cytoplasmic IFN-γ. In these cell lines, large proportions of CD4+ and CD8+ cells coexpressed LAG-3 and CD30, a putative marker of Th2-like cells. Overall, our data do not support the earlier suggestion that LAG-3 and CD30 are selective markers of T cells with type-1 and type-2 cytokine profiles, respectively.