MHC II (Major Histocompatibility Complex class II) molecules are cell-surface proteins that present processed antigenic epitopes to CD4+ T helper cells. Expression on antigen-presenting cells (APCs) like macrophages, dendritic cells, and B cells is essential for initiating adaptive immune responses.
After macrophages phagocytose and break down pathogens, they display epitope fragments (pathogen 'passports') on MHC II molecules at the cell surface. This MHC II-epitope complex is recognized by CD4+ T cell receptors (TCR). The quality and quantity of MHC II expression determines the strength of adaptive immune activation. Mature 'ripe' macrophages have high MHC II expression and produce IL-12 and IL-2, driving Th1 responses. Immature or 'unripe' macrophages have minimal MHC II expression, resulting in weak or inappropriate adaptive responses.
Inadequate MHC II expression on macrophages results in poor antigen presentation, leading to weak adaptive immunity, increased infection susceptibility, and potential autoimmunity (when self-antigens are presented instead). This occurs when macrophages remain semi-mature or become 'unripe' by consuming host tissue during chronic inflammation, creating a vicious cycle of immune dysfunction.
- Expressed on antigen-presenting cells (macrophages, dendritic cells, B cells)
- Presents processed epitopes to CD4+ T helper cells
- Expression level determines strength of adaptive immune activation
- Resting macrophages have minimal MHC II expression
- Mature 'ripe' macrophages dramatically upregulate MHC II
- Poor MHC II presentation leads to weak adaptive immunity
- Immature macrophages with low MHC II may present self-antigens inappropriately
- MHC II-epitope binding to TCR is essential for T cell activation
- Macrophages — primary antigen-presenting cells expressing MHC II after phagocytosis
- CD4+ T cells — recognize and bind to MHC II-epitope complexes via T cell receptor
- epitopes — pathogen fragments displayed on MHC II molecules for T cell recognition
- Antigen spreading — inadequate MHC II presentation can lead to autoimmunity through inappropriate self-antigen presentation
- IL-12 — mature macrophages with high MHC II produce IL-12, driving Th1 differentiation
- IL-2 — ripe macrophages produce IL-2 alongside MHC II upregulation, supporting T cell proliferation
- Th1 — MHC II presentation with IL-12 drives Th1 cell-mediated immunity
- adaptive immunity — MHC II presentation bridges innate and adaptive immune systems
- Dendritic cells — professional APCs with constitutive MHC II expression, more potent than macrophages
- B cells — express MHC II to present antigens to T helper cells for antibody production
- phagocytosis — precedes antigen processing and MHC II presentation
- Efferocytosis — macrophages eating apoptotic cells downregulate MHC II, preventing autoimmunity
- M1 macrophages — inflammatory M1 macrophages upregulate MHC II for robust antigen presentation
- M2 macrophages — resolution M2 macrophages downregulate MHC II as inflammation resolves
- chronic inflammation — chronic inflammation can impair MHC II function through macrophage dysfunction
- Autoimmunity — inappropriate MHC II presentation of self-antigens triggers autoimmune responses
- HLA — HLA class II is the human form of MHC II
- T cell receptor — TCR on CD4+ T cells binds to MHC II-epitope complex for activation
- Cytotoxic T cells — CD8+ cells recognize MHC I, distinct from CD4+ recognition of MHC II
- vaccine — effective vaccines depend on MHC II presentation to generate T helper responses