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The human genome encodes thousands of genes that regulate immune recognition, tolerance, and response. Among these, the Human Leukocyte Antigen (HLA) complex, located on chromosome 6p21, has the most significant impact on autoimmune susceptibility. The HLA system, also known as the Major Histocompatibility Complex (MHC), encodes proteins responsible for antigen presentation. These proteins bind fragments of foreign or self-peptides and display them on the cell surface for inspection by T cells. Variations in HLA genes can modify peptide-binding affinity, altering the immune system’s ability to distinguish between self and non-self.
Specific HLA alleles have strong disease associations:
HLA-B27: Found in 90–95% of patients with ankylosing spondylitis, it influences T-cell receptor recognition and promotes inflammatory cytokine release.
HLA-DRB1 shared epitope: A sequence motif common in rheumatoid arthritis (RA) that enhances the presentation of self-peptides to CD4⁺ T cells.
HLA-DR3 and DR2: Linked with systemic lupus erythematosus (SLE) and multiple sclerosis, suggesting a role in defective self-tolerance mechanisms.
HLA-DQ2/DQ8: Strongly associated with celiac disease, allowing the presentation of gluten-derived peptides that trigger intestinal inflammation.
Beyond HLA: The Role of Non-HLA Genetic Factors
Although HLA accounts for a substantial proportion of heritable risk, it does not fully explain autoimmune susceptibility. Genome-wide association studies (GWAS) have identified numerous non-HLA genes that modulate immune pathways. Key examples include:
- PTPN22: Encodes a lymphoid tyrosine phosphatase that regulates T-cell activation. The R620W variant leads to overactive T-cell signaling, contributing to diseases like T1D, RA, and SLE.
- CTLA4: Produces a checkpoint protein that inhibits T-cell responses. Polymorphisms reducing CTLA4 expression or function are associated with autoimmune thyroiditis and type 1 diabetes.
- IL2RA (CD25): Encodes a component of the interleukin-2 receptor essential for regulatory T-cell (Treg) function. Variants can impair Treg-mediated tolerance.
- STAT4 and IRF5: Transcription factors that amplify cytokine signaling; both are linked to lupus and inflammatory arthritis.
- TNFAIP3 (A20): Regulates the NF-κB pathway, a major inflammatory cascade. Mutations can lead to uncontrolled cytokine production and persistent inflammation.
Gene Environment Interaction: When Genetics Meets Lifestyle and Infection
Viral and bacterial infections
Pathogens such as Epstein–Barr virus, Coxsackievirus, or Mycobacterium tuberculosis can induce molecular mimicry, where microbial antigens resemble self-proteins. This confusion can provoke cross-reactive immune responses.
Hormonal factors
The higher prevalence of autoimmune diseases in women (approximately 80% of cases) underscores the influence of estrogen and other hormones in modulating immune activity.
Smoking and toxins
Smoking has been shown to alter HLA-related risk in rheumatoid arthritis by promoting the formation of citrullinated proteins, leading to the production of anti-citrullinated peptide antibodies (ACPA).
Diet and microbiota
The gut microbiome shapes immune education; dysbiosis (microbial imbalance) can enhance intestinal permeability and promote systemic inflammation.
Ultraviolet light and stress
UV exposure can damage DNA and release self-antigens, while stress hormones modulate cytokine networks and immune cell activation.
The Epigenetic Layer: Modifying Gene Expression Without Changing DNA
Epigenetics bridges the gap between genes and the environment. It refers to reversible chemical modifications that regulate gene expression without altering the underlying DNA sequence. The main epigenetic mechanisms include:
- DNA methylation: The addition of methyl groups to cytosine bases can silence genes. In lupus, hypomethylation of DNA in T cells leads to overexpression of immune-activating genes such as CD11a and CD70.
- Histone modification: Acetylation and methylation of histone proteins affect chromatin accessibility. Abnormal histone marks are seen in diseases like SLE and RA, influencing cytokine gene expression.
- MicroRNAs (miRNAs): These small non-coding RNAs regulate gene translation. Dysregulated miRNAs (e.g., miR-155, miR-146a) are known to enhance inflammatory responses and reduce immune tolerance.
Epigenetic patterns are dynamic and can change with aging, infections, diet, or drug exposure, explaining the variability of autoimmune disease onset even among genetically identical individuals.