Antigen presentation is a mechanism by which cells enable their recognition by T cells through displaying antigens of either endogenous or exogenous origin in the context of MHC molecules . There are two types of MHC molecules, MHC class I and MHC class II. MHC class I molecules are expressed on all nucleated cells, while MHC class II molecules are expressed solely on professional antigen presenting cells, such as Dendritic cells (DCs) . The basis of adaptive immunity lies within our cells’ ability to present foreign antigens on MHC class I molecules to T cells, signifying to the T cell that it has been mutated or infected, leading to the cell-mediated killing of that cell . The role of DCs is to activate the adaptive immune system. They do this by internalizing foreign antigens and then presenting them on either MHC class I molecules, which are recognized by cytotoxic T cells (CTLs) or on MHC class II molecules which are recognized by T helper cells, leading to their activation . There are three routes of antigen presentation. The first is called the endogenous (MHC class I) route, the second is called the exogenous (MHC class II) route and the last is called cross-presentation .
The endogenous route of antigen presentation involves MHC class I molecules which present peptides from intracellular origin to the immune system [1,2]. Successful presentation of intracellular peptides on MHC class I molecules involves a series of sequential steps resulting in the expression of a foreign (or mutated, in the case of tumor cells) peptides in the context of MHC I molecules on the surface of the cell [1,2]. It all starts with the ubiquitination of protein substrates for their degradation by the proteasome [1,2]. Proteins which are tagged are either stable proteins nearing the end of their life or misfolded and/or mistranslated proteins that do not display the wild type characteristics of their endogenous form . MHC I molecules are present on all nucleated cells within the human body . The reason behind their expression is to allow for the detection of cells which exhibit abnormal expression patterns by the immune system. This process is called “immunosurveillence”.
Once the proteins are degraded by the proteasome in the cytoplasm, they are translocated into the endoplasmic reticulum (ER) through the TAP protein channel [1,2]. Inside the ER, they are complexed with MHC class I molecules with the help of chaperone proteins which regulate their assembly into the peptide binding cleft of the MHC molecule [1,2]. Once the peptides have bound to MHC, they are directed in vesicles from the ER to the golgi and ultimately to the plasma membrane where the vesicles fuse, leading to the presentation of the MHC I:peptide complex on the surface of the cell [1,2]. When on the surface of the cell, the MHC I:peptide complex can be actively surveyed by incoming immune cells which will ultimately determine the fate of that cell [1,2]. It is important to note that in the context of cancer, the nucleated cell may be a cancer cell that is expressing mutated antigens on MHC I molecules.
In the exogenous pathway of antigen presentation, MHC class II molecules are assembled in the ER and paired with the invariant chain, CD74 [1,2]. The role of CD74 is to act as a “plug” for the peptide-binding cleft of the MHC II molecule, preventing endogenous peptide assembly in the ER [1,2]. Once it has been assembled, the MHC II molecule + invariant chain complex is guided from the ER to the peptide loading compartment [1,2]. The peptides present in this loading compartment are of extracellular origin. For this to happen, extracellular antigens are endocytosed into endolysosomes and degraded by the action of proteases and high pH present in the endolysosome [1,2]. The degraded proteins, now called ‘peptides’ are able to associate with MHC II molecules [1,2]. Due to the high pH of the endolysosome, the invariant chain dissociates from the peptide-binding cleft, thereby allowing for peptide binding [1,2]. Once peptides have been successfully loaded on MHC II molecules, the complexes leave the peptide-loading compartment and travel to the plasma membrane via vesicles, where fusion facilitates their surface expression [1,2].
It is important to remember that MHC class II expression is unique to professional antigen presenting cells, such as DCs . Perhaps the most notable of these cells is the Dendritic cell. In the context of cancer, a DC may endocytose live or dead tumor cells and/or their respective tumor-associated antigens from the extracellular space, and present their antigens on MHC II via the aforementioned process. Activation of T helper cells requires an “immunological synapse”, a process where T cell receptors (TCRs) expressed on T helper cells synergize with MHC II:peptide complexes present on the surface of the DC .
Cytotoxic T cells have TCRs which recognize antigens in the context of MHC I molecules . This poses a problem for the DC, whose role is to prime the CTLs, activating them against antigens of extracellular origin. To overcome this dilemma, the DC employs a mechanism known as cross-presentation. Cross-presentation is a method for DCs to present the antigenic universe without being consumed in the process. That is, it provides a way by which the DC can express tumor-associated antigen of extracellular origins on MHC I molecules (which are the only type of MHC molecules recognized by CTLs) [1,4]. Dying cancer cells are an extensively rich source of antigens that are taken up by DCas and help stimulate cross-presentation .
Cross-presentation occurs when a DC endocytoses a tumor antigen (or a tumor cell) and directs it to the peptide loading compartment where the antigen will be degraded into peptides that can fit the peptide-binding cleft of the MHC molecule [1,3]. Simultaneously, MHC class I molecules associate with the invariant chain in the ER . A localization signal present on the invariant chain directs the MHC I molecule to the peptide loading compartment for exogenous peptide loading . Subsequent dissociation of the invariant chain allows exogenous peptides to be loaded on MHC I molecules . It is important to appreciate the importance of cross-presentation in the generation of an appropriate immune response. Without this mechanism of CTL priming, our bodies would not be able to mount a proper cell-mediated anti-tumor response.
From the point of view of a tumor cell, it would be desirable to prevent both endogenous antigen presentation as well as DC-mediated activation of adaptive immune cells via the exogenous pathway and cross-presentation. Effective anti-tumor CTL responses occur only if there is sufficient MHC I expression on the surface of tumor cells . In recent literature, numerous cancer types have been described to have MHC I expression abnormalities . The implications of reduced MHC I expression by the vast majority of tumor cells include a highly reduced immune response against cancer cells, since CTL immunosurveillence is impeded . By reducing their endogenous antigen expression, tumor cells effectively evade detection.
In addition to preventing their own detection by CTLs, tumor cells employ methods of blocking antigen cross-presentation in dendritic cells. Dysfnctional dendritic cells that cannot carry out cross-presentation and CTL priming have been extensively described in patients with multiple cancers . These DCs were shown to have a reduced ability to activate a robust immune response. Cao et al. have most recently elucidated a mechanism by which tumor cells inhibit cross-presentation in DCs. Their studies show that oxidized lipids, secreted by tumor cells, blocked the cross-presentation of exogenous antigens to CTLs without inhibiting the endogenous antigen presentation of the DC (an important distinction to make because if this process was inhibited in the DC it would induce cell death) .
In recent years, two main methods of have been employed in order to combat the problem of antigen expression in cancer: adoptive Dendritic cell transfer and viral-delivered TAP expression.
Adoptive DC therapy uses a patient’s own dendritic cells for therapeutic vaccination against cancer cells . Adoptive DC therapy is performed in a series of sequential steps aimed at promoting an increase in tumor-specific DCs within the patient. The general method of adoptive DC transfer begins with the harvest and isolation of a dendritic cell population from the patient’s blood . These cells are then either pulsed with tumor antigens or transfected with a viral vector . The cells are then allowed to proliferate in vitro to increase their numbers prior to re-introducing them back into the patient . Upon transfusion into the patient, the newly generated population of activated DCs present tumor-associated antigens on their surface in the context of MHC I and II molecules, allowing for a robust stimulation of a CTL-mediated immune response . Dendritic cell vaccines have already been implemented within the clinical setting after receiving FDA approval in recent years .
In order to increase the intrinsic immunogenicity of the tumor cell itself, a mechanism which employs the delivery of the TAP gene using a viral vector has been implemented experimentally in order to upregulate MHC I expression . TAP is the protein channel which transports the peptides into the ER to be loaded onto MHC I molecules, and has been characterized to be the downregulated in many types of cancers . It has been shown that viral delivery of the TAP gene into tumor cells has been proven to be efficacious in increasing the MHC I expression on tumor cells and boosting their general immunogenicity .
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