HLA Associations and Antigen-Specific Regulatory T Cells in Autoimmune Disease

Research Timeline and Speaker's Expertise * The presenter has dedicated nearly ten years to researching the association between Human Leukocyte Antigen (HLA) and autoimmune diseases. * The primary goal of this research is to establish a definitive link between HLA associations and the concept of antigen-specific regulatory T cells (Tregs). * Historically, the discovery of thymus function in the 1960s coincided with the observation that individuals with autoimmune diseases often inherit specific HLA genes that predispose them to these conditions.
Genetic Risk Examples (Janeway's Textbook) * Type 1 Diabetes: Individuals who inherit the alleles DQ2 or DQ8 from their parents face a risk that is statistically $25$ times higher than the general population. * Goodpasture’s Syndrome (also referred to as Gilpinj’s disease in the transcript): * This is a rare condition, affecting approximately one in a million individuals ( $1 : 1,000,000$ ). * It is characterized by aggressive, cell-mediated immunity directed against a specific kidney antigen. * This immune response leads to the rapid destruction of the kidney within a short timeframe. * The disease is highly valuable for immunological study due to its clear HLA association (single HLA type) and its single autoepitope. * Individuals expressing the DR2 (or DR15) phenotype have a risk factor that is $15$ times higher than average.

Statistical Analysis of Goodpasture’s Syndrome * A forest plot of three separate studies indicates an average odds ratio of $8.5$ for developing the disease. * While DR15 is associated with high susceptibility, the inheritance of DR1 provides a protective effect, significantly reducing the likelihood of disease development.
Demographic Distribution and Phenotypes * General Population: * Approximately $10\%$ of the general population carries the DR15 allele. * Approximately $10\%$ of the general population carries the DR1 allele. * Goodpasture’s Patient Population: * Between $80\%$ and $90\%$ of patients are DR15 positive. * Fewer than $1$ in $100$ (less than $1\%$ ) of patients are DR1 positive. * This distribution raises a critical research question: How does DR15 increase susceptibility while DR1 actively protects against the disease?

Tetramer Detection of T Cells * Researchers synthesized tetramers to present the specific Goodpasture autoepitope, which was defined as a sequence of $12$ amino acids fitting into the peptide-binding group of the MHC molecule. * This tool allowed the detection of antigen-specific T cells in several groups: * Patients with the disease. * Healthy human subjects expressing DR15 or DR1. * DR-transgenic mice.
Comparison of Cell Types Based on HLA Status * DR15 (High-Risk) Humans: * The majority of antigen-specific T cells were found to be pro-inflammatory. * These cells were identified as FOXP3 negative. * DR1 (Protective) Humans: * The majority of cells were CD4 positive and FOXP3 positive. * In humans, these cells were further defined by being CD127 low. * These individuals demonstrated a "dominant protection" over DR15.
Quantitative T-Cell Ratios * DR15 Individual Ratio: 1 Regulatory T cell (Treg) to every 10 T conventional cells (Tcon) ( $1 : 10$ ). * DR1 Individual Ratio: 10 Regulatory T cells (Treg) to every 1 T conventional cell (Tcon) ( $10 : 1$ ). * Consequently, a DR1 individual possesses $100$ times as many regulatory T cells relative to T conventional cells compared to a DR15 individual. * In individuals homozygous for both DR15 and DR1, the DR15 tetramer still identifies more T conventional cells, while the DR1 tetramer identifies more regulatory T cells.

Peptide Binding Pocket Polymorphism * Variations between different DR types are caused by differences in the peptide-binding pocket of the beta chain. * Highly variable amino acids in the beta chain affect how peptides bind and how they are presented to T cells.
Crystal Structure Findings (Rostron Lab and Synchrotron Data) * Researchers solved the crystal structure of the Goodpasture epitope located at alpha three, residues $135$ to $145$ . * Visual components of the structure include: * Alpha chain: Green. * Beta-15 chain: Blue. * Goodpasture peptide: Purple. * DR15 Binding Dynamics: The aromatic rings of the peptide fit snugly into the P4 and P6 pockets. They are buried within the pocket, creating a relatively flat structure on top. * DR1 Binding Dynamics (MHC Overlay): Due to different pocket shapes in DR1 (DRB1*01), a "register shift" occurs. This shift forces the aromatic rings out of the pockets, creating a more protruding, "pokey out" peptide structure.

TCR Binding and Affinity * The structural differences likely alter the T cell receptor (TCR) binding affinity. * The "pokey out" aromatic ring structure in DR1 leads to a stronger interaction with the TCR, resulting in higher affinity. * Higher affinity during presentation in the thymus preferentially generates regulatory T cells. * The flat structure in DR15 results in lower affinity binding, leading to the generation of T conventional cells.
Conclusions * Inheriting specific HLA types can directly protect against or predispose an individual to autoimmune diseases. * The conformation of the peptide-HLA complex determines TCR binding affinity in the thymus, which is a critical factor in the development of regulatory T cells. * Antigen-specific Tregs provide dominant protection against pro-inflammatory autoreactive responses in both humans and disease models of Goodpasture’s syndrome.
Learning Objectives Checklist * Define a Regulatory T cell (Treg), noting the main markers: CD25 and FOXP3 (the master transcription factor). * Understand the distinction between N Tregs (Natural) and I Tregs (Induced). * Describe the mechanisms by which Tregs effect immunosuppression. * Link observed HLA associations in autoimmune disease to the development of Tregs in the thymus based on peptide-HLA presentation and activation in the periphery.