Microbes and protein antigens that enter through epithelia are concentrated in lymph nodes, and blood-borne antigens are captured mostly in the spleen (Fig.1). The common routes through which foreign antigens, such as microbes, enter a host are the skin and the epithelia of the gastrointestinal and respiratory systems. In addition, microbial antigens may be produced in tissues that have been colonized by microbes. The skin, mucosal epithelia, and parenchymal organs contain numerous lymphatic capillaries that drain lymph from these sites and into the regional lymph node. Some antigens are transported in the lymph by APCs (primarily DCs) that capture the antigen and enter lymphatic vessels, and other antigens enter the lymphatics in cell-free form. Thus, the lymph contains a sampling of all the soluble and cell-associated antigens that enter through epithelia and are present in tissues. The antigens become concentrated in lymph nodes into which the lymph flows. Lymph nodes are interposed along lymphatic vessels and act as filters that sample the lymph before it reaches the blood. Antigens that enter the bloodstream may be sampled by APCs that are resident in the spleen, which has a rich blood supply.
Fig1. Routes of antigen entry. Microbial antigens commonly enter through the skin and gastrointestinal and respiratory tracts, where they are captured by dendritic cells and transported to regional lymph nodes. Antigens that enter the bloodstream are captured by antigen-presenting cells in the spleen.
DCs that are resident in epithelia and tissues capture protein antigens. Tissue-resident conventional DCs (cDCs) express numerous membrane receptors, such as C-type lectins, that bind microbes. DCs use these receptors to capture and endocytose microbes or microbial proteins and then process the ingested proteins into peptides capable of binding to MHC molecules. In addition to receptor-mediated endocytosis and phagocytosis, DCs can ingest antigens by pinocytosis, a process that does not involve specific receptors but serves to internalize whatever molecules might be in the fluid phase in the vicinity of the DCs.
Simultaneously with antigen capture, DCs are activated by microbial products to mature into APCs that transport the captured antigens to draining lymph nodes (Fig.2). At the time that microbial antigens are being captured, microbial products (i.e., pathogen-associated molecular patterns [PAMPs]), different from the protein antigens that T cells recognize, are recognized by Toll-like receptors and other innate pattern recognition receptors in the DCs and other cells, generating innate immune responses. The DCs are activated by these signals and by cytokines, such as tumor necrosis factor (TNF), produced in response to the microbes. The activated DCs (also called mature DCs) lose their adhesiveness for epithelia or connective tissue and begin to express a chemokine receptor called CCR7 that is specific for two chemokines, CCL19 and CCL21, which are produced in lymphatic vessels and in the T-cell zones of lymph nodes. These chemokines attract the DCs bearing microbial antigens into draining lymphatics and ultimately into the T-cell zones of the regional lymph nodes. Naive T cells also express CCR7, and this is why they localize to the same regions of lymph nodes where antigen-bearing DCs are concentrated, although their route into the lymph node is via the blood. The colocalization of antigen-bearing activated DCs and naive T cells maximizes the chance of T cells with receptors for the antigen finding the antigen they can recognize.
Fig2. Role of dendritic cells (DCs) in antigen capture and presentation. (A) Immature DCs in the skin (Langerhans cells) or dermis capture antigens that enter through the epidermis and transport the antigens to regional lymph nodes. During this migration, the DCs mature and become efficient antigen-presenting cells. (B) The table summarizes some of the changes during DC maturation that are important in the functions of these cells. Half-life is an estimate of how long the molecules are expressed on cells. The number of surface molecules is per class II–expressing cell. ICAM-1, Intercellular adhesion molecule 1; IL-12, interleukin-12; MHC, major histocompatibility complex.
Activation also converts the DCs from cells whose primary function is to capture antigen into cells that are able to present antigens to naive T cells and to activate the lymphocytes. Activated DCs express high levels of MHC molecules with bound peptides and costimulators required for T-cell activation. Thus, by the time these cells arrive in the lymph nodes, they have developed into potent APCs with the ability to activate T lymphocytes. Naive T cells that recirculate through lymph nodes encounter these APCs, and the T cells that are specific for the displayed peptide-MHC complexes are activated. This is the initial step in the induction of T-cell responses to protein antigens.
In the absence of infection or inflammation, conventional DCs capture antigens in the tissues but few migrate to lymph nodes and they are not activated to produce the high levels of cytokines and costimulators that are required to induce effective immune responses. The function of these DCs may be to present self antigens to self-reactive T cells and thereby cause inactivation or death of the T cells or generate regulatory T cells. These mechanisms play a role in maintaining self-tolerance and preventing autoimmunity.
Antigens are also transported to lymphoid organs in soluble form. Resident DCs in the lymph nodes and spleen may capture lymph- and blood-borne antigens, respectively, and also may be driven to mature by microbial products. When lymph enters a lymph node through an afferent lymphatic vessel, it drains into the subcapsular sinus, and some of the lymph enters fibroblast reticular cell (FRC) conduits that originate from the sinus and traverse the cortex. Once in the conduits, low molecular-weight antigens can be extracted by DCs that line the outside surfaces of the conduits and whose processes interdigitate between the FRCs. Other antigens in the subcapsular sinus are taken up by macrophages, which carry the antigens into follicles and present these antigens to resident B cells. B cells in the node may also recognize and internalize soluble antigens.
The collection and concentration of foreign antigens in lymph nodes are supplemented by other anatomic adaptations that serve similar functions. The mucosal surfaces of the GI and respiratory systems, in addition to being drained by lymphatic capillaries, contain specialized collections of secondary lymphoid tissue that can directly sample the luminal contents of these organs for the presence of antigenic material. The best characterized of these mucosal lymphoid organs are Peyer’s patches of the ileum and the pharyngeal tonsils. APCs in the spleen monitor the bloodstream for any anti gens that reach the circulation. Such antigens may reach the blood either directly from the tissues or by way of the lymph from the thoracic and right lymphatic ducts.
Several properties of conventional DCs make them the most efficient APCs for initiating primary T-cell responses.
• DCs are strategically located at the common sites of entry of microbes and foreign antigens (in epithelia) and in tissues that may be colonized by microbes.
• DCs express receptors that enable them to capture and respond to microbes.
• In response to chemokines, activated DCs migrate from epithelia and tissues via lymphatics, preferentially into the T-cell zones of lymph nodes, the same regions of the lymph nodes through which naive T lymphocytes also circulate.
• Mature DCs express high levels of peptide-MHC complexes, costimulators, and cytokines, all of which are needed to activate naive T lymphocytes.
• Specialized DCs (cDC1) can transfer internalized proteins from phagosomes into the cytosol and are thus efficient at cross-presenting antigens to CD8+ T cells. As we will see later, this process is essential for initiating CD8+ T-cell responses to many viruses and tumors.
