Lymphocyte activation gene-3 : the expression and function in the immune system

Lymphocyte activation gene-3 (LAG-3) is the structural homologue of a well-known
TCR co-receptor CD4. Recently, it has been suggested that LAG-3 might play a role in
modulation of immune response, by being a negative regulator of T cell activation.
To further characterize its expression and function in the immune system, monoclonal
Abs were generated against mouse LAG-3. The availability of different mAbs allowed
the identification of LAG-3 on various immune subpopulations.
The main finding of this work is that, unlike previously thought, LAG-3 expression is not
limited to activated T and NK cells, but can also be induced on B cells and DCs. B cells
were found to express LAG-3 in a T cell dependent manner; LAG-3 was expressed on B
cells in the presence of activated T cells, but not upon stimulation with thymus
independent stimuli like CpG or LPS. Furthermore, requirements for LAG-3 induction
on B cells were defined; this event requires B cells proliferation and is mediated by a
soluble factor released by activated T cells.
Among DCs, two populations, CD8- (‘myeloid’) and plasmacytoid DCs, expressed high
levels of surface LAG-3 upon stimulation with LPS and CpGs, respectively.
To evaluate the potential role of LAG-3 in APC function, LAG-3 deficient and OT-II
transgenic mice were used. Antigen presentation assay showed, that LAG-3 presence on
the surface of APC can affect T cell responses. T cells stimulated with OVA peptide
presented by LAG-3 deficient B cells proliferated poorly compared to T cell stimulated
with OVA-pulsed LAG-3 positive B cells. This finding suggests a novel co-stimulatory
function of LAG-3 on APCs.
In addition, experiments re-evaluating published data concerning the role of LAG-3 on T
cells were performed. In agreement with previous reports, the ectopic expression of
LAG-3 on the surface of T cell line exerted an inhibitory effect on T cell activation
induced by relevant antigen. However, in disagreement with recent report (by Huang et
al. 2004), the standard regulatory cell assays performed with normal as well as LAG-3
deficient CD4+CD25+ cells, suggested no involvement of LAG-3 in regulatory T cell
function.
Furthermore, the experiments demonstrating the surface expression on murine CD4 and
CD8 T cells in vivo are presented. LAG-3 was identified on homeostatically expanding T
cells after transfer into lymphopenic mice.
To investigate structural aspects of the LAG-3 molecule, soluble LAG-3 (sLAG-3),
composed of four extracellular domains was produced. For this purpose Drosophila
expression system was employed. The conditions were developed for production of two
forms of sLAG-3, the original glycosylated form and its non-glycosylated version.
Biochemical analysis of soluble protein showed that in solution LAG-3 forms stable
dimers, which suggests the presence of such dimers on cell surface. Because
heterogenous glycosylation of WT form likely prevents protein crystallization, attempts
are now made to crystallize non-glycosylated form of sLAG-3 alone or in complex with
its ligand, MHC class II. The detailed structural information would significantly enhance
our knowledge about LAG-3 and allow for rational drug design, if desired.