Trogocytosis (Greek : trogo; gnaw) is when a cell nibbles another cell. [1] It is a process whereby lymphocytes (B, T and NK cells) conjugated to antigen-presenting cells extract surface molecules from these cells and express them on their own surface. [2] The molecular reorganization occurring at the interface between the lymphocyte and the antigen-presenting cell during conjugation is also called "immunological synapse".
First indication for the existence of this process dates back late 70s when several research groups reported on the presence of unexpected molecules such as Major Histocompatibility complex molecules (MHC) on T cells. The notion that membrane fragments, and not isolated molecules, could be captured by T cells on antigen-presenting cells was suggested by the capture of MHC molecules fused to the green fluorescent protein (GFP) in their intracellular portion. [3] The demonstration that membrane fragments were involved in this transfer process came when fluorescent probes incorporated in the plasma membrane of the antigen-presenting cell as well as non-MHC molecules were found to be captured by T cells together with the antigen. [4] [5]
Trogocytosis has been initially documented in T, B, and NK cells both in vivo and in vitro. On T cells and B cells, trogocytosis is triggered when the T cell receptor (TCR) on T cells or B cell receptor (BCR) on B cells interacts with the antigen recognized on antigen-presenting cells. Like in lymphocytes, trogocytosis occurs with PMN (polymorphonuclear leukocytes, also known as granulocytes) and is associated with effective ADCC (Antibody dependent cell mediated cytotoxicity).
It was shown that in order to initiate ADCC in vitro, PMN's have to adhere to their target cells and form tight junctions with antibody opsonized tumor cells. This cell clustering precedes mutual membrane lipid exchange between effector and target cell during ADCC and does not happen in the absence of opsonizing antibodies. [6] Trogocytosis also occurs in monocytes, and dendritic cells. Outside the immune system, similar transfer of membrane fragments have been documented between sperm and oocytes, a process thought to contribute to gamete fusion. [7]
Lately the term has been attributed to macrophages, such as the CNS resident microglia, which are able to partially remove small portions of neuronal axons during postnatal development. [8]
Trogocytosis involves the transfer of plasma membrane fragments from the presenting cell to the lymphocyte. Trogocytosis is specifically triggered by antigen receptor signalling on T and B cells, by killer inhibitory and killer activatory receptor on NK cells and by various receptors on other cells including Fc receptor and scavenger class A receptor. It is likely that trogocytosis does not involve the capture of vesicles such as exosomes secreted by antigen-presenting cells. Rather, molecules could move from antigen-presenting cells to lymphocytes conveyed by membrane nanotubes or membrane fragments could be torn by T cells due to physical forces required for immunological synapse formation and deformation. Depending on the two cell types involved in conjugates, trogocytosis can be unidirectional or bidirectional. Proteins transferred by trogocytosis are many and mostly include proteins inserted in or closely associated to the plasma membrane (proteins spanning the lipid bilayer or inserted in the extracellular or intracellular leaflets). For instance, human lymphocytes were recently shown to acquire the inner-membrane protein H-Ras, a G-protein vital for common lymphocyte functions and a prominent participant in human cancer, from the cells they scan. [9] The transfer was cell contact-dependent and occurred in the context of cell-conjugate formation. Moreover, the acquisition of oncogenic H-RasG12V by NK- and T lymphocytes had important biological functions in the adopting lymphocytes: the transferred H-RasG12V induced ERK phosphorylation, increased interferon-γ and tumor necrosis factor-α secretion, enhanced lymphocyte proliferation, and augmented NK-mediated target cell killing.
Trogocytosis can have physiological consequences in two ways: either because "recipient" cells acquire and make use of molecules they do not usually express or because «donor» cells are stripped of molecules, which may alter their interaction with cellular partners. Acquired molecules, such as regulatory molecules with extracellular or intracellular components might alter the lymphocytes activity and direct several lymphocyte functions, such as migration to the adequate injured tissues. Such gained plasma membrane fragments could also contribute to the capacity to proliferate, because lipids are highly energetic claiming components to establish. Trogocytosis might have appeared first in very primitive organisms to feed off other cells. Most of the biological functions identified for trogocytosis have been reported for lymphocytes and dendritic cells. Major findings along these lines are:
Therapeutic antibodies can be used to treat cancer. An example is rituximab, a therapeutic antibody used to treat chronic lymphocytic leukemia, recognizes the CD20 molecule expressed by tumor cells and leads to their elimination. [15] However, using too much of the antibody results in part from the removal of rituximab-CD20 complexes from the tumor cell surface by monocytes through trogocytosis. This effect leads to tumors cell escape by antigenic modulation. Reducing the dose of therapeutic antibodies to limit the extent of trogocytosis might improve their therapeutic efficacy. [16]
Epratuzumab (a CD22 Mab) acts using trogocytosis to transfer CD22 and other B-cell proteins from B cells to effector cells. [17]
TRAP assays (TRogocytosis Analysis Protocol) allow to identify, characterize and purify T and B cells recognizing their specific antigen based on their ability to extract molecules (in that case, fluorescent probes) from the plasma membrane of antigen-presenting cells. [18] These assays require equipment such as a flow cytometer but are otherwise very cheap, easy to perform, fast (can be performed within 3 hours) and applicable to any population of T or B cells. TRAP assays have been successfully used to detect T cell responses against viral infections, [19] cancer, [20] autoimmune diseases [21] and vaccines. [22]
The process of Trogocytosis is considered different from but similar to the unrelated processes known as Phagocytosis and Paracytophagy.
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.
A DNA vaccine is a type of vaccine that transfects a specific antigen-coding DNA sequence into the cells of an organism as a mechanism to induce an immune response.
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 inserted into the plasma membrane where they serve as a part of B-cell receptors. When a naïve or memory B cell is activated by an antigen, it proliferates and differentiates into an antibody-secreting effector cell, known as a plasmablast or plasma cell. Additionally, B cells present antigens and secrete cytokines. In mammals, B cells mature in the bone marrow, which is at the core of most bones. In birds, B cells mature in the bursa of Fabricius, a lymphoid organ where they were first discovered by Chang and Glick, which is why the 'B' stands for bursa and not bone marrow as commonly believed.
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 called MHC molecules.
Cancer immunotherapy is the stimulation of the immune system to treat cancer, improving on the immune system's natural ability to fight the disease. It is an application of the fundamental research of cancer immunology and a growing subspecialty of oncology.
Antigen processing, or the cytosolic pathway, is an immunological process that prepares antigens for presentation to special cells of the immune system called T lymphocytes. It is considered to be a stage of antigen presentation pathways. This process involves two distinct pathways for processing of antigens from an organism's own (self) proteins or intracellular pathogens, or from phagocytosed pathogens ; subsequent presentation of these antigens on class I or class II major histocompatibility complex (MHC) molecules is dependent on which pathway is used. Both MHC class I and II are required to bind antigens before they are stably expressed on a cell surface. MHC I antigen presentation typically involves the endogenous pathway of antigen processing, and MHC II antigen presentation involves the exogenous pathway of antigen processing. Cross-presentation involves parts of the exogenous and the endogenous pathways but ultimately involves the latter portion of the endogenous pathway.
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 their T cell receptors (TCRs). APCs process antigens and present them to T-cells.
The T-cell receptor (TCR) is a protein complex found on the surface of T cells, or T lymphocytes, that is responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. The binding between TCR and antigen peptides is of relatively low affinity and is degenerate: that is, many TCRs recognize the same antigen peptide and many antigen peptides are recognized by the same TCR.
Cross-presentation is the ability of certain professional antigen-presenting cells (mostly dendritic cells) to take up, process and present extracellular antigens with MHC class I molecules to CD8 T cells (cytotoxic T cells). Cross-priming, the result of this process, describes the stimulation of naive cytotoxic CD8+ T cells into activated cytotoxic CD8+ T cells. This process is necessary for immunity against most tumors and against viruses that infect dendritic cells and sabotage their presentation of virus antigens. Cross presentation is also required for the induction of cytotoxic immunity by vaccination with protein antigens, for example, tumour vaccination.
The following are notable events in the Timeline of immunology:
Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment can be recognized by a T-cell receptor. Specifically, the fragment, bound to the major histocompatibility complex (MHC), is transported to the surface of the cell, a process known as presentation. If there has been an infection with viruses or bacteria, the cell will present an endogenous or exogenous peptide fragment derived from the antigen by MHC molecules. There are two types of MHC molecules which differ in the behaviour of the antigens: MHC class I molecules (MHC-I) bind peptides from the cell cytosol, while peptides generated in the endocytic vesicles after internalisation are bound to MHC class II (MHC-II). Cellular membranes separate these two cellular environments - intracellular and extracellular. Each T cell can only recognize tens to hundreds of copies of a unique sequence of a single peptide among thousands of other peptides presented on the same cell, because an MHC molecule in one cell can bind to quite a large range of peptides. Predicting which antigens will be presented to the immune system by a certain MHC/HLA type is difficult, but the technology involved is improving.
MHC Class II molecules are a class of major histocompatibility complex (MHC) molecules normally found only on professional antigen-presenting cells such as dendritic cells, mononuclear phagocytes, some endothelial cells, thymic epithelial cells, and B cells. These cells are important in initiating immune responses.
B-lymphocyte antigen CD20 or CD20 is expressed on the surface of all B-cells beginning at the pro-B phase and progressively increasing in concentration until maturity.
The Cluster of differentiation 80 is a B7, type I membrane protein in the immunoglobulin superfamily, with an extracellular immunoglobulin constant-like domain and a variable-like domain required for receptor binding. It is closely related to CD86, another B7 protein (B7-2), and often works in tandem. Both CD80 and CD86 interact with costimulatory receptors CD28, CTLA-4 (CD152) and the p75 neurotrophin receptor.
CD83 is a human protein encoded by the CD83 gene.
Gamma delta T cells are T cells that have a γδ T-cell receptor (TCR) on their surface. Most T cells are αβ T cells with TCR composed of two glycoprotein chains called α (alpha) and β (beta) TCR chains. In contrast, γδ T cells have a TCR that is made up of one γ (gamma) chain and one δ (delta) chain. This group of T cells is usually less common than αβ T cells, but are at their highest abundance in the gut mucosa, within a population of lymphocytes known as intraepithelial lymphocytes (IELs).
HLA class I histocompatibility antigen, alpha chain F is a protein that in humans is encoded by the HLA-F gene. It is an empty intracellular molecule that encodes a non-classical heavy chain anchored to the membrane and forming a heterodimer with a β-2 microglobulin light chain. It belongs to the HLA class I heavy chain paralogues that separate from most of the HLA heavy chains. HLA-F is localized in the endoplasmic reticulum and Golgi apparatus, and is also unique in the sense that it exhibits few polymorphisms in the human population relative to the other HLA genes; however, there have been found different isoforms from numerous transcript variants found for the HLA-F gene. Its pathways include IFN-gamma signaling and CDK-mediated phosphorylation and removal of the Saccharomycescerevisiae Cdc6 protein, which is crucial for functional DNA replication.
Lymphocyte-activation gene 3, also known as LAG-3, is a protein which in humans is encoded by the LAG3 gene. LAG3, which was discovered in 1990 and was designated CD223 after the Seventh Human Leucocyte Differentiation Antigen Workshop in 2000, is a cell surface molecule with diverse biologic effects on T cell function. It is an immune checkpoint receptor and as such is the target of various drug development programs by pharmaceutical companies seeking to develop new treatments for cancer and autoimmune disorders. In soluble form it is also being developed as a cancer drug in its own right.
CD160 antigen is a protein that in humans is encoded by the CD160 gene.
Kinetic-segregation is a model proposed for the mechanism of T-cell receptor (TCR) triggering. It offers an explanation for how TCR binding to its ligand triggers T-cell activation, based on size-sensitivity for the molecules involved. Simon J. Davis and Anton van der Merwe, University of Oxford, proposed this model in 1996. According to the model, TCR signalling is initiated by segregation of phosphatases with large extracellular domains from the TCR complex when binding to its ligand, allowing small kinases to phosphorylate intracellular domains of the TCR without inhibition. Its might also be applicable to other receptors of the Non-catalytic tyrosine-phosphorylated receptors family such as CD28.