Can APCs present self-antigens to T cells?

– The simple answer is yes! APCs can present self-antigens to T cells. Self-antigens are molecules that are normally present in the body and do not trigger an immune response. However, under certain conditions, such as infection, inflammation, or autoimmunity, APCs can process and present self-antigens to T cells, which may result in self-reactive or regulatory T cells.

– There are different mechanisms by which APCs can present self-antigens to T cells. One mechanism is direct presentation, which occurs when APCs express self-antigens on their own MHC molecules and interact with T cells in the same tissue or organ. This can lead to tolerance or activation of T cells, depending on the costimulatory signals and cytokines involved.

– Another mechanism is cross-presentation, which occurs when APCs take up self-antigens from dying or damaged cells and present them on MHC class I molecules to CD8+ cytotoxic T cells. This can induce immune responses against tumors or virus-infected cells, but also against healthy cells in some autoimmune diseases.

– A third mechanism is cross-dressing, which occurs when APCs acquire intact MHC-peptide complexes from other cells and display them on their surface without processing them. This can allow APCs to present antigens from distant tissues or organs to T cells in the lymph nodes or spleen. This can also induce immune responses against tumors or virus-infected cells, but also against healthy cells in some autoimmune diseases.

– A fourth mechanism is MHC class II-dressing, which occurs when APCs acquire intact MHC class II-peptide complexes from other APCs and display them on their surface without processing them. This can allow APCs to present antigens from different sources to CD4+ helper T cells. This can modulate the immune response by enhancing or suppressing the activation of helper T cells.

– APCs play a crucial role in presenting self-antigens to T cells and regulating the balance between tolerance and immunity. However, this process can also be dysregulated and contribute to immune-mediated diseases such as autoimmunity, allergy, and transplant rejection.

What is the role of T cells in antigen presentation?

– T cells are a type of lymphocyte that play a key role in the adaptive immune system. They can recognize and respond to antigens, which are molecules that trigger an immune response, such as proteins from pathogens or tumors. However, T cells cannot directly recognize antigens. They need the help of antigen-presenting cells (APCs), which are immune cells that can process and present antigens to T cells.

– There are two main types of T cells: CD4+ helper T cells and CD8+ cytotoxic T cells. Each type of T cell has a different function and interacts with a different type of APC.

– CD4+ helper T cells; are involved in activating and regulating other immune cells, such as B cells, macrophages, and other T cells. They can recognize antigens presented by MHC class II molecules, which are molecules that display peptides from extracellular sources, such as bacteria, toxins, or allergens. MHC class II molecules are expressed by specialized APCs, such as dendritic cells, macrophages, and B cells.

– CD8+ cytotoxic T cells; are involved in killing infected or abnormal cells, such as virus-infected cells or tumor cells. They can recognize antigens presented by MHC class I molecules, which are molecules that display peptides from intracellular sources, such as viral proteins or tumor antigens. MHC class I molecules are expressed by all nucleated cells in the body, but they can also be acquired by some APCs through cross-presentation or cross-dressing mechanisms.

– The process of antigen presentation involves several steps that can be detailed as:

• The APC takes up the antigen through endocytosis or phagocytosis and degrades it into peptides in endosomes or lysosomes. The APC loads the peptides onto MHC molecules in the endoplasmic reticulum (ER) or endosomal compartments. The loading is facilitated by various proteins, such as invariant chain (Ii), transporter associated with antigen processing (TAP), or tapasin.

• The peptide-MHC complexes are transported to the plasma membrane of the APC, where they can be recognized by T cell receptors (TCRs) on the surface of T cells. The APC also provides costimulatory signals and cytokines to the T cell, which are essential for the activation and differentiation of the T cell.

Antigen presentation is a vital process that allows T cells to "see" what proteins are present in the body and to form an adaptive immune response against them. It also enables the immune system to distinguish between self and non-self antigens and to prevent autoimmune reactions.