MHC Class II
   HOME

TheInfoList



OR:

MHC Class II molecules are a class of
major histocompatibility complex The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are calle ...
(MHC) molecules normally found only on professional antigen-presenting cells such as
dendritic cell Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. ...
s, mononuclear phagocytes, some
endothelial cell The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...
s,
thymic The thymus is a specialized primary Lymphatic system#Structure, lymphoid organ of the immune system. Within the thymus, T cell, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body ada ...
epithelial cells, and
B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...
s. These cells are important in initiating
immune responses An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...
. The
antigens In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
presented by class II
peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A ...
s are derived from extracellular proteins (not cytosolic as in
MHC class I MHC class I molecules are one of two primary classes of major histocompatibility complex (MHC) molecules (the other being MHC class II) and are found on the cell surface of all nucleated cells in the bodies of vertebrates. They also occur on plat ...
). Loading of a MHC class II molecule occurs by
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
; extracellular proteins are endocytosed, digested in
lysosome A lysosome () is a membrane-bound organelle found in many animal cells. They are spherical vesicles that contain hydrolytic enzymes that can break down many kinds of biomolecules. A lysosome has a specific composition, of both its membrane prot ...
s, and the resulting epitopic peptide fragments are loaded onto MHC class II molecules prior to their migration to the
cell surface The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
. In humans, the MHC class II protein complex is encoded by the human leukocyte antigen gene complex (HLA). HLAs corresponding to MHC class II are
HLA-DP HLA-DP is a protein/peptide-antigen receptor and graft-versus-host disease antigen that is composed of 2 subunits, DPα and DPβ. DPα and DPβ are encoded by two loci, HLA-DPA1 and HLA-DPB1, that are found in the MHC Class II (or HLA-D) regio ...
,
HLA-DM HLA-DM (human leukocyte antigen DM) is an intracellular protein involved in the mechanism of antigen presentation on antigen presenting cells (APCs) of the immune system. It does this by assisting in peptide loading of major histocompatibility co ...
,
HLA-DOA HLA class II histocompatibility antigen, DO alpha chain is a protein that in humans is encoded by the ''HLA-DOA'' gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredi ...
,
HLA-DOB HLA class II histocompatibility antigen, DO beta chain is a protein that in humans is encoded by the ''HLA-DOB'' gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredit ...
,
HLA-DQ HLA-DQ (DQ) is a cell surface receptor protein found on antigen-presenting cells. It is an αβ heterodimer of type MHC class II. The α and β chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromos ...
, and
HLA-DR HLA-DR is an MHC class II cell surface receptor encoded by the human leukocyte antigen complex on chromosome 6 region 6p21.31. The complex of HLA-DR (Human Leukocyte Antigen – DR isotype) and peptide, generally between 9 and 30 amino acids in ...
. Mutations in the HLA gene complex can lead to
bare lymphocyte syndrome Bare lymphocyte syndrome is a condition caused by mutations in certain genes of the major histocompatibility complex or involved with the processing and presentation of MHC molecules. It is a form of severe combined immunodeficiency. Presentati ...
(BLS), which is a type of MHC class II deficiency.


Structure

Like
MHC class I MHC class I molecules are one of two primary classes of major histocompatibility complex (MHC) molecules (the other being MHC class II) and are found on the cell surface of all nucleated cells in the bodies of vertebrates. They also occur on plat ...
molecules, class II molecules are also
heterodimer In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' has ...
s, but in this case consist of two homogenous peptides, an α and β chain, both of which are encoded in the MHC. The subdesignation α1, α2, etc. refers to separate domains within the HLA gene; each domain is usually encoded by a different exon within the gene, and some genes have further domains that encode leader sequences, transmembrane sequences, etc. These molecules have both extracellular regions as well as a transmembrane sequence and a cytoplasmic tail. The α1 and β1 regions of the chains come together to make a membrane-distal peptide-binding domain, while the α2 and β2 regions, the remaining extracellular parts of the chains, form a membrane-proximal immunoglobulin-like domain. The antigen binding groove, where the antigen or peptide binds, is made up of two α-helixes walls and β-sheet. Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer, generally between 15 and 24
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
residues long.


Expression

These molecules are constitutively expressed in professional, immune
antigen-presenting cell An antigen-presenting cell (APC) or accessory cell is a cell that displays antigen bound by major histocompatibility complex (MHC) proteins on its surface; this process is known as antigen presentation. T cells may recognize these complexes using ...
s, but may also be induced on other cells by
interferon γ Interferon gamma (IFN-γ) is a dimerized soluble cytokine that is the only member of the type II class of interferons. The existence of this interferon, which early in its history was known as immune interferon, was described by E. F. Wheelock ...
. They are expressed on the epithelial cells in the thymus and on APCs in the periphery. MHC class II expression is closely regulated in APCs by
CIITA CIITA is a human gene which encodes a protein called the class II, major histocompatibility complex, transactivator. Mutations in this gene are responsible for the bare lymphocyte syndrome in which the immune system is severely compromised and c ...
, which is the MHC class II transactivator. CIITA is solely expressed on professional APCs however, non-professional APCs can also regulate CIITA activity and MHC II expression. As mentioned interferon γ (IFN γ ) triggers the expression of CIITA and is also responsible for converting
monocytes Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also infl ...
which are MHC class II negative cells into functional APCs that express MHC class II on their surfaces. MHC class II is also expressed on group 3
innate lymphoid cell Innate lymphoid cells (ILCs) are the most recently discovered family of innate immune cells, derived from common lymphoid progenitors (CLPs). In response to pathogenic tissue damage, ILCs contribute to immunity via the secretion of signalling mo ...
s.


Importance

Having MHC class II molecules present proper peptides that are bound stably is essential for overall immune function. Because class II MHC is loaded with extracellular proteins, it is mainly concerned with presentation of extracellular pathogens (for example, bacteria that might be infecting a wound or the blood). Class II molecules interact mainly with immune cells, like the
T helper cell The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considere ...
( CD4+). The peptide presented regulates how T cells respond to an infection. Stable peptide binding is essential to prevent detachment and degradation of a peptide, which could occur without secure attachment to the MHC molecule. This would prevent T cell recognition of the antigen, T cell recruitment, and a proper immune response. The triggered appropriate immune response may include localized
inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...
and swelling due to recruitment of phagocytes or may lead to a full-force antibody immune response due to activation of
B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...
s.


Synthesis

During synthesis of class II MHC in the endoplasmic reticulum, the α and β chains are produced and complexed with a special polypeptide known as the
invariant chain HLA class II histocompatibility antigen gamma chain also known as HLA-DR antigens-associated invariant chain or CD74 (Cluster of Differentiation 74), is a protein that in humans is encoded by the ''CD74'' gene. The invariant chain (Abbreviated Ii ...
. The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular peptides or peptides from the endogenous pathway (such as those that would be loaded onto class I MHC). The invariant chain also facilitates the export of class II MHC from the ER to the
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...
, followed by fusion with a late endosome containing endocytosed, degraded proteins. The invariant chain is then broken down in stages by proteases called
cathepsins Cathepsins (Ancient Greek ''kata-'' "down" and ''hepsein'' "boil"; abbreviated CTS) are proteases (enzymes that degrade proteins) found in all animals as well as other organisms. There are approximately a dozen members of this family, which are di ...
, leaving only a small fragment known as
CLIP Clip or CLIP may refer to: Fasteners * Hair clip, a device used to hold hair together or attaching materials such as caps to the hair * Binder clip, a device used for holding thicker materials (such as large volumes of paper) together ** Bulldog ...
which maintains blockage of the peptide binding cleft on the MHC molecule. A MHC class II-like structure,
HLA-DM HLA-DM (human leukocyte antigen DM) is an intracellular protein involved in the mechanism of antigen presentation on antigen presenting cells (APCs) of the immune system. It does this by assisting in peptide loading of major histocompatibility co ...
, facilitates
CLIP Clip or CLIP may refer to: Fasteners * Hair clip, a device used to hold hair together or attaching materials such as caps to the hair * Binder clip, a device used for holding thicker materials (such as large volumes of paper) together ** Bulldog ...
removal and allows the binding of peptides with higher affinities. The stable class II MHC is then presented on the cell surface.


Recycling of MHC class II complexes

After MHC class II complexes are synthesized and presented on APCs they are unable to be expressed on the cell surface indefinitely, due to the internalization of the
plasma membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
by the APCs(antigen presenting cells). In some cells, antigens bind to recycled MHC class II molecules while they are in the early
endosomes Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membrane can ...
, while other cells such as dendritic cells internalize antigens via receptor-mediated endocytosis and create MHC class II molecules plus peptide in the endosomal-lysosomal antigen processing compartment which is independent of the synthesis of new MHC class II complexes. These suggest that after the antigen is internalized, already existent MHC class II complexes on mature
dendritic cells Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. The ...
can be recycled and developed into new MHC class II molecules plus peptide.


Antigen processing and presentation

Unlike MHC I, MHC II is meant to present extracellular pathogens rather than intracellular. Furthermore, the first step is to acquire the pathogen through phagocytosis. The pathogen is then broken down in a lysosome and a desired component is then acquired and loaded onto a MHC II molecule. The MHC II molecule then travels to the surface to present the antigen to a helper T cell. MHC II active helper T cells which help release cytokines and other things which will help induce other cells which help to combat the pathogens outside the cells.


Genes


Pathways controlling MHC class II antigen presentation


Pathway: PSD4–ARL14/ARF7–MYO1E


Molecules involved

Several molecules are involved in this pathway. *PIK3R2 and PIP5K1A are two kinases that create substrates for PSD4. *PSD4 (Pleckstrin and Sec7 Domain containing 4) is a
GEF Gef ( ), also referred to as the Talking Mongoose or the Dalby Spook, was the name given to an allegedly talking mongoose which was claimed to inhabit a farmhouse owned by the Irving family. The Irvings' farm was located at Cashen's Gap near ...
(Guanine nucleotide Exchange Factor) that loads ARL14/ARF7 with GTP. *ARL14/ARF7 is a
Small GTPase Small GTPases (), also known as small G-proteins, are a family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). They are a type of G-protein found in the cytosol that are homologous to the alpha subunit of heterotri ...
protein that is selectively expressed in immune cells. This protein is localized within MHC-II compartments in immature
dendritic cell Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. ...
s. *ARF7EP is an effector of ARL14/ARF7 that interacts with
MYO1E Myosin-Ie (Myo1e) is a protein that in humans is encoded by the ''MYO1E'' gene. Myosin-Ie is a long tailed myosin. It contains an N-terminal motor domain, an IQ motif, a TH1 domain containing a plecstrin homology (PH) domain, a proline rich TH2 ...
. *MYO1E is a protein that controls MHC-II compartments with an actin-based mechanism.


Pathway

PIK3R2 and PIP5K1A are two
kinases In biochemistry, a kinase () is an enzyme that catalysis, catalyzes the transfer of phosphate groups from High-energy phosphate, high-energy, phosphate-donating molecules to specific Substrate (biochemistry), substrates. This process is known as ...
that phosphorylate
Phosphatidylinositol Phosphatidylinositol (or Inositol Phospholipid) consists of a family of lipids as illustrated on the right, where red is x, blue is y, and black is z, in the context of independent variation, a class of the phosphatidylglycerides. In such molecul ...
(PIP) providing PSD4 with substrates for its GTP loading ability. PSD4 as a guanine exchange factor, loads ARL14/ARF7 with GTP. Subsequently, ARF7EP interacts with
MYO1E Myosin-Ie (Myo1e) is a protein that in humans is encoded by the ''MYO1E'' gene. Myosin-Ie is a long tailed myosin. It contains an N-terminal motor domain, an IQ motif, a TH1 domain containing a plecstrin homology (PH) domain, a proline rich TH2 ...
which binds itself to
actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
myofibers. Altogether, this complex contributes to maintain MHC-II loaded vesicles within the immature
dendritic cell Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. ...
, impeding its translocation to the cell membrane.


Bare lymphocyte syndrome

One type of MHC class II deficiency, also called bare lymphocyte syndrome, is due to mutations in the genes that code for transcription factors that regulate the expression of the MHC class II genes. It results in the depletion of CD4 T cells and some immunoglobulin isotypes even though there are normal levels of both
CD8 CD8 (cluster of differentiation 8) is a transmembrane glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). Along with the TCR, the CD8 co-receptor plays a role in T cell signaling and aiding with cytotoxic T cell-antigen int ...
Cells and
B cells B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...
present. Deficient MHC class II molecules are unable to present antigens to T cells and properly activate T cells. T cells are then unable to proliferate, and secrete
cytokines Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...
which normally participate in the immune response. Not only do the deficient MHC class II molecules affect the activation and proliferation of T cells but also the rest of the immune response cascade which includes B cells. Therefore, with this decrease in the number of T cells, the T cells cannot interact and activate the B cells. Normally when B cells are activated they divide, proliferate and differentiate, which includes the differentiation of these cells into plasma cells which are responsible for producing antibodies. However, when there is a deficiency in MHC class II molecules B cells are not activated and cannot differentiate into
plasma cells Plasma cells, also called plasma B cells or effector B cells, are white blood cells that originate in the lymphoid organs as B lymphocytes and secrete large quantities of proteins called antibodies in response to being presented specific substan ...
which causes them to be deficient in
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
which are unable to perform as they are expected. The only current form of treatment is a bone-marrow transplant however even this does not cure the disease and most patients do not live past age ten.


MHC class II and Type I diabetes

MHC class II genes and molecules are related to a multitude of different diseases, one of which being
Type I diabetes Type 1 diabetes (T1D), formerly known as juvenile diabetes, is an autoimmune disease that originates when cells that make insulin (beta cells) are destroyed by the immune system. Insulin is a hormone required for the cells to use blood sugar for ...
. HLA class II genes are the most important genes associated with the risk of inheriting Type I diabetes, accounting for about 40-50% of
heritability Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of h ...
.
Alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
of these genes that affect peptide binding to the MHC class II molecules seem to impact Type I diabetes risk the most. Specific allele polymorphisms have been identified to increase the risk (such as DRB1 and DQB1). Others have been associated with a resistance to the disease.


See also

* Cross-presentation *
Bare lymphocyte syndrome Bare lymphocyte syndrome is a condition caused by mutations in certain genes of the major histocompatibility complex or involved with the processing and presentation of MHC molecules. It is a form of severe combined immunodeficiency. Presentati ...


References


External links

* * {{Surface antigens Genes Immune system Glycoproteins