Inhibition of thymocyte positive selection by natural MHC: peptide ligands

Barcoding T cell calcium response diversity with methods for automated and accurate analysis of cell signals (MAAACS)

We introduce a series of experimental procedures enabling sensitive calcium monitoring in T cell populations by confocal video-microscopy. Tracking and post-acquisition analysis was performed using Methods for Automated and Accurate Analysis of Cell Signals (MAAACS), a fully customized program that associates a high throughput tracking algorithm, an intuitive reconnection routine and a statistical platform to provide, at a glance, the calcium barcode of a population of individual T-cells. Combined with a sensitive calcium probe, this method allowed us to unravel the heterogeneity in shape and intensity of the calcium response in T cell populations and especially in naive T cells, which display intracellular calcium oscillations upon stimulation by antigen presenting cells.

The adaptive immune response to pathogen invasion requires the stimulation of lymphocytes by antigen-presenting cells. We hypothesized that investigating the dynamics of the T lymphocyte activation by monitoring intracellular calcium fluctuations might help explain the high specificity and selectivity of this phenomenon. However, the quantitative and exhaustive analysis of calcium fluctuations by video microscopy in the context of cell-to-cell contact is a tough challenge. To tackle this, we developed a complete solution named MAAACS (Methods for Automated and Accurate Analysis of Cell Signals), in order to automate the detection, cell tracking, raw data ordering and analysis of calcium signals. Our algorithm revealed that, when in contact with antigen-presenting cells, T lymphocytes generate oscillating calcium signals and not a massive and sustained calcium response as was originally thought. We anticipate our approach providing many more new insights into the molecular mechanisms triggering adaptive immunity.

Altered positive selection due to corecognition of floppy peptide/MHC II conformers supports an integrative model of thymic selection

Thymocytes bearing the E alpha 52-68/I-A(b) complex-specific 1H3.1 alpha beta T cell antigen receptor are positively selected in Ab-Ep [Ab-Ep transgenic, invariant chain (Ii)(-/-), I-A beta(b-/-)] mice, where I-A(b) molecules present only E alpha 52-68. Although Ii reintroduction led to deletion, I-A beta(b) reintroduction disrupted positive selection. T cell antigen receptor transgenic Ab-Ep I-A beta(b+) mice had a large thymus with an increased absolute number of CD4(+)CD8(+) cells and no overt signs of deletion. Unlike Ab-Ep Ii(+) antigen-presenting cells, Ab-Ep I-A beta(b+) antigen-presenting cells did not activate 1H3.1 T cells. However, their capacity to present E alpha 52-68 was intact. Thus, positive selection of 1H3.1 thymocytes on the tight compact E alpha 52-68/I-A(b) complex is neutralized by the corecognition of loose compact self-peptide/I-A(b) conformers that do not interfere with the cognate activation of mature 1H3.1 T cells. The data support the notion that the integration of distinct signals generated by the simultaneous recognition of multiple self-peptide/MHC complexes directs intrathymic selection of T cells.

Major histocompatibility complex class II presentation of cell-associated antigen is mediated by CD8alpha+ dendritic cells in vivo

Antigen-specific B cells express major histocompatibility complex class II and can present antigen directly to T cells. Adoptive transfer experiments using transgenic B and T cells demonstrated that antigen-specific B cells can also efficiently transfer antigen to another cell for presentation to T cells in vivo. To identify the antigen-presenting cell that receives antigens from B cells, a strategy was developed to follow the traffic of B cell-derived proteins in vivo. B cells were labeled with the fluorescent dye CFSE and loaded with antigen, before adoptive transfer into recipient mice. Populations of splenocytes from the recipient mice were later assayed for the presence of fluorescent proteins and for the ability to activate T cells. A small number of CD8alpha+CD4-CD11b(lo) dendritic cells (DCs) contain proteins transferred from B cells and these DCs effectively present antigens derived from the B cells to T cells. The results suggest that CD8alpha+ DCs sample the cells and membranes in their environment for presentation to T cells circulating through the T cell zone. This function of CD8alpha+ DCs may be relevant to the priming of an immune response or the induction of T cell tolerance.

The level of peptide-MHC complex determines the susceptibility to autoimmune diabetes: studies in HEL transgenic mice

We report a mouse model for the spontaneous development of autoimmune diabetes: the 3A9 T cell receptor (TCR) transgenic mouse, which contains T cells that recognize the 52 - 61 family of hen egg-white lysozyme (HEL) peptides in the context of MHC class II I-A(k) molecules, was bred to the ILK3 mouse, that expresses HEL protein via the rat insulin promoter (RIP). Despite partial tolerance of 3A9 T cells in ILK3 mice, spontaneous diabetes developed in 64 % of 3A9xILK3 mice by 20 weeks of age. We provide evidence that APC from peri-pancreatic nodes have a large content of peptide-MHC complex and stimulate 3A9 T cells. We also report that cross presentation of HEL from beta cells to APC is 26-fold more efficient than presentation of soluble HEL. We previously reported on a biochemical margin of safety, based on the observation that activation of naive 3A9 T cells required 100-fold more peptide-MHC complexes than required for deletion of 3A9 thymocytes. We speculate that the high local density of autologous peptide-MHC complexes can be a determining factor that leads to the activation of autoreactive CD4 T cells and, consequently, to the development of autoimmunity.

Lineage fate alteration of thymocytes developing in an MHC environment containing MHC/peptide ligands with antagonist properties

A18 TCR transgenic thymocytes which are H-2E(k) restricted and normally selected into the CD4 lineage, are exclusively selected into the CD8 lineage in an H-2(q) MHC background. CD8 T cell selection in the H-2(q) background is far more efficient than default selection of A18 CD8 cells on a CD4(-/-) H-2E(k +) background. This suggests the involvement of special selecting ligands. Analogues of the cognate peptide for A18 with antagonist properties for the A18 TCR have previously been shown to effect a lineage diversion from CD4 to CD8 in fetal thymic organ cultures and intriguingly the MHC(q) background contains unidentified natural MHC class II ligands which similarly show antagonist properties for the A18 TCR. Despite the presence of these unidentified MHC class II ligands in the H-2(q) background and their potential influence on developing A18 thymocytes, however, MHC class I molecules were essential for thymic selection of A18 CD8 T cells.

Quantitative analysis of the T cell repertoire that escapes negative selection

Mice expressing hen egg-white lysozyme (HEL) as a transgene are unresponsive to immunization with the HEL protein. Profound tolerance was found even in situations where the amounts of l-A(k)-peptide complexes was 100 or less per APC. Among the few T cells that escaped tolerance, we did not observe differential responses to the different HEL epitopes, perhaps because of the very high sensitivity of the negative selection process. The same HEL transgenic mice that did not respond to HEL responded to immunization with the 46-61 peptide of HEL. These peptide-specific T cells that escaped negative selection belonged to a set that reacted with a particular conformer of the HEL peptide-l-A(k) (type B). The presence of type B reactive T cells should be considered in autoimmunity.