MHC Class II — Human Engineering

Overview

MHC (Major Histocompatibility Complex) class II molecules are encoded in the MHC locus on chromosome 6p21 (the HLA region). In humans, there are three isotypes: HLA-DR (most immunologically dominant; highest surface density on APCs), HLA-DQ (strong association with type 1 diabetes: DQB1*03:02/DQA1*03:01, and celiac disease: DQ2/DQ8), and HLA-DP (lower surface density; associated with beryllium sensitivity and some malignancies).

The key structural distinction from MHC class I is that the class II peptide-binding groove is open at both ends, allowing it to accommodate longer peptides (9–25 aa, average 15 aa) derived from exogenous proteins degraded in the endosomal/lysosomal pathway. MHC class II is recognized exclusively by CD4⁺ T cells, whose CD4 co-receptor binds the non-polymorphic β2 domain of the MHC-II molecule.

Distinction from MHC class I: MHC-I (HLA-A, -B, -C) presents endogenously-derived peptides (~8–10 aa) to CD8⁺ cytotoxic T cells. MHC-II presents exogenous peptides to CD4⁺ T helpers. This division of labor — intracellular vs. extracellular threats — underlies much of adaptive immunity.

Structure

Component	Gene / Size	Key Features
α chain	DRA (~229 aa); relatively non-polymorphic	α1 + α2 domains; α1 contributes one floor and one α-helix of the peptide-binding groove; α2 is membrane-proximal Ig-fold domain
β chain	DRB1, DQB1, DPB1; highly polymorphic	β1 + β2 domains; β1 forms the other half of the groove; polymorphism concentrated in α-helices flanking the groove — determines peptide repertoire and alloreactivity; β2 contacts CD4 co-receptor (non-polymorphic)
Peptide-binding groove	Open at both ends (unlike class I)	Nine binding pockets (P1–P9) with anchor residues; P1 (β57) and P9 (β1/β11) are dominant anchors; DQ8 has Ala at β57 → opens P9 → different peptide repertoire vs. DQ2 (Ser57) → determines autoimmune peptide specificity in T1D and celiac disease
β2 domain	Non-polymorphic	Binds CD4 co-receptor on T helper cells → co-localizes Lck → lowers TCR signaling threshold; equivalent role to the β2m/α3 contact for CD8 on MHC class I

Antigen Presentation Pathway (MIIC Pathway)

MHC class II presents peptides from the exogenous (endosomal/lysosomal) pathway, distinct from the proteasomal TAP pathway used by MHC class I.

Endocytosis: Extracellular proteins (pathogens, soluble antigens, allergens) are endocytosed into early endosomes → late endosomes → lysosomes, where cathepsin B, D, and S proteases degrade them to peptides of 9–25 aa.
MHC II synthesis and invariant chain (Ii, CD74): In the ER, newly assembled MHC-II αβ heterodimers are associated with the invariant chain. The CLIP region of Ii occupies the peptide-binding groove (preventing premature peptide loading) and the cytoplasmic tail of Ii contains lysosomal-targeting signals (LLXXXL motif) → directs the MHC II–Ii complex to the MIIC (MHC class II compartment / multivesicular late endosome).
CLIP removal by HLA-DM: In the acidic MIIC, cathepsin S clips Ii to leave only the CLIP fragment in the groove. HLA-DM (a non-classical MHC-II heterodimer that does not reach the cell surface) acts as a peptide editor/chaperone — catalyzes CLIP displacement and the exchange of low-affinity peptides for high-affinity peptides → stabilizes the most tightly binding peptide-MHC-II complexes.
HLA-DO regulation: HLA-DO is a negative regulator of HLA-DM; expressed predominantly in B cells → modulates the peptide repertoire presented by B cells vs. DCs.
Surface trafficking: Stable peptide-MHC-II complexes are transported via tubular lysosomes to the plasma membrane → presented to CD4⁺ T cells; half-life of surface MHC-II–peptide complexes is 24–48 hours on mature DCs.

Cross-presentation exception: Dendritic cells can also load exogenous antigen onto MHC class I (cross-presentation), critical for priming CD8⁺ responses against viruses and tumors — but this involves distinct intracellular routing into the cytoplasm/proteasomal pathway, not the MIIC pathway.

CD4⁺ T Cell Recognition

T cell receptor (TCR) contacts both MHC-II α-helices simultaneously and the bound peptide nestled between them. The CD4 co-receptor stabilizes this contact by binding to the β2 domain of MHC-II (non-polymorphic region) → co-localizes ZAP70 and Lck → lowers the TCR signaling threshold by ~100-fold compared with signaling without CD4.

The lifetime (dwell time) of the peptide-MHC-II–TCR ternary complex determines T cell fate:

Too short (<~1 sec): No productive signaling → T cell remains naive or ignorant
Productive range (~1–100 sec): NFAT, AP-1, NF-κB activation → T cell activation, differentiation into Th1/Th2/Th17/Treg depending on cytokine milieu
Too long (very high affinity): Can promote anergy or deletion (negative selection in thymus)

The immunological synapse (IS) further organizes signaling: ICAM-1 on APCs engages LFA-1 on T cells → central cSMAC (TCR, CD4, PKCθ, ZAP70) surrounded by peripheral pSMAC (LFA-1/ICAM-1) → sustained signaling over minutes to hours.

Expression Regulation

Constitutive Expression

Dendritic cells (highest surface density of any APC)
B cells (all mature B cells; upregulated on activation)
Macrophages (lower than DCs; further upregulated by IFN-γ)
Thymic cortical and medullary epithelium (essential for positive and negative selection)

Inducible Expression (via IFN-γ)

IFN-γ → JAK1/2 → STAT1 phosphorylation → CIITA (MHC class II transactivator) transcription. CIITA is the master regulator — not a direct DNA-binding factor but a co-activator that recruits the RFX complex (RFX5, RFXANK/B, RFXAP) to the X-box/W-box/Y-box promoter elements of HLA-DR, HLA-DQ, HLA-DP, invariant chain, and HLA-DM genes simultaneously.

IFN-γ-inducible MHC-II expression occurs in epithelial cells, endothelial cells, fibroblasts, and even activated T cells — expanding the antigen-presenting network during inflammation.

Negative Regulation

IL-10, TGF-β, glucocorticoids → suppress CIITA → reduce MHC-II expression
Viral immunoevasins: HCMV US2/US3 primarily target class I; EBV BNLF2a; HSV ICP47 — several viruses also downregulate class II to evade CD4⁺ T helper surveillance
Tumor microenvironment: IL-10, PD-L1, IDO suppress DC maturation and MHC-II upregulation

Pathology

Condition	Mechanism	Key Features	Management
Bare Lymphocyte Syndrome Type II	Loss-of-function mutations in CIITA, RFX5, RFXANK, or RFXAP → complete absence of MHC-II expression on all cells	Severe combined immunodeficiency; recurrent bacterial, viral, fungal, protozoal infections; failure to thrive; mortality in first decade without treatment	Hematopoietic stem cell transplantation (HSCT); gene therapy investigational
Type 1 Diabetes (T1D)	HLA-DR3/4 + DQ2/DQ8 haplotypes: aberrant peptide binding promotes autoreactive CD4⁺ T cell priming against islet antigens (insulin, GAD65, IA-2)	Strongest genetic risk factor for T1D (>50% of genetic susceptibility); DQ8 Ala57 at β chain opens P9 pocket → favors presentation of insulinogenic peptides	Insulin replacement; teplizumab (anti-CD3) delays onset in at-risk individuals
Rheumatoid Arthritis (RA)	HLA-DR4 "shared epitope" (amino acids 67–74 of DRβ1): same sequence in DRB104:01, 04:04, *01:01 → preferential presentation of citrullinated peptides → anti-CCP antibodies + synovial CD4⁺ T cell activation	3–5× increased RA risk; associated with more severe, erosive disease	DMARDs (methotrexate), biologics (TNF-i, IL-6Ri, abatacept)
Celiac Disease	HLA-DQ2 (DQA105/DQB102) or DQ8 (DQA103/DQB103:02): deamidated gliadin peptides bind DQ2/DQ8 with high affinity → robust CD4⁺ Th1 response in lamina propria	>95% of celiac patients carry HLA-DQ2 or DQ8; necessary but not sufficient (only ~3% of HLA-DQ2 carriers develop celiac); villous atrophy, malabsorption	Gluten-free diet; larazotide, nexvax2 (investigational)
Organ Transplant Rejection	Donor-recipient MHC-II mismatch → alloreactive CD4⁺ T cells recognize foreign HLA-DR/DQ/DP directly or via indirect presentation; anti-HLA antibodies → complement-mediated humoral rejection	HLA-DR matching is most critical for kidney transplant survival; HLA-DQ mismatch increasingly recognized as major driver of chronic AMR	HLA matching; calcineurin inhibitors; anti-CD20 (rituximab) for DSA
Superantigen Activation	S. aureus TSST-1, staphylococcal enterotoxins bind outside MHC-II peptide groove → activate all T cells bearing certain Vβ chains (up to 20% of T cells simultaneously) → cytokine storm	Toxic shock syndrome (TSS); massive IL-2, IFN-γ, TNF release; multi-organ failure	Supportive; IVIG (neutralizes superantigen); clindamycin (reduces toxin production)

Connections

PresenterB Cell — professional APC; MHC-II on B cells presents antigen to T helper cells, enabling T-dependent antibody responses
PresenterMHC Class II (extended) — related entry with additional structural and genetic detail
T CellImmune System — MHC-II is the central molecular bridge between innate phagocytosis and adaptive T cell response

References

Neefjes J, et al. Towards a systems understanding of MHC class I and MHC class II antigen presentation. Nat Rev Immunol. 2011;11:823–836.
Unanue ER. Perspective on antigen processing and presentation. Immunol Rev. 2002;185:86–102.
Pittet MJ, et al. HLA associations in autoimmune and infectious diseases: comprehensive review. Nat Immunol. 2022.