Dendritic cells and keratinocytes have key roles in the sensitization phase

Antigen presenting cells (APC) in the skin include Langerhans’ cell (LCs), located in the suprabasal epidermis, and dermal dendritic cells (dDCs). Contact hypersensitivity is primarily an epidermal reaction, and epidermal LCs were considered to be the APC responsible for initiating contact sensitivity (Fig. 26.3). More recent studies have established that dDCs are essential for stimulating hapten-specific T cells.

Fig. 26.3 Langerhans’ cells

(1) These dendritic cells constitute 3% of all cells in the epidermis. They express a variety of surface markers, including Langerin and CD1. Here they have been identified in normal skin using an anti-CD1 monoclonal antibody (counterstained with Mayer’s hemalum). (L, Langerhans’ cell; K, keratinocyte.) × 312. (2) Electron micrograph of a Langerhans’ cell showing the characteristic ‘Birbeck granule’. This organelle is a platelike structure derived from cell membranes, often with a bleb-like extension at one end. × 132 000.

Langerhans’ cells (see Chapter 2) are specialized DCs which extend dendritic processes throughout the epidermis, allowing them to sample environmental antigens. LCs express MHC class II, CD1 and the C-type lectin, langerin (CD207), which is responsible for the development of Birbeck granules, the cell membrane-derived organelle characteristic of LCs (see Fig. 26.3). The majority of dermal DCs are Langerin⁻, but there is a small population of Langerin⁺ dDCs, which are distinct from LCs, but also migrate rapidly to draining lymph nodes on exposure to sensitizers and activate hapten-specific CD8⁺ T cells. Both LCs and dDCs take up hapten-modified proteins by micropinocytosis but they also absorb lipid-soluble haptens, which modify cytoplasmic proteins. Under the influence of IL-1 and TNF secreted by keratinocytes and other cells, these DCs undergo maturation and increase expression of MHC and co-stimulatory molecules. Both LCs and dDCs are inactivated by ultraviolet B, which can therefore prevent or alleviate the effects of contact hypersensitivity.

Keratinocytes produce cytokines important to the contact hypersensitivity response

Keratinocytes provide the structural integrity of the epidermis and have a central role in epidermal immunology. Keratinocytes can be activated by a number of stimuli, including sensitizing agents and irritants. They may express MHC class II molecules and intercellular adhesion molecule-1 (ICAM-1) in the cell membrane.

Activated keratinocytes produce a wide range of cytokines, including:

Sensitization stimulates a population of memory T cells

Sensitization takes 10–14 days in humans. Hapten-bearing LCs and dDCs bearing modified proteins migrate as veiled cells through the afferent lymphatics to the paracortical areas of regional lymph nodes, where they activate CD4⁺ and CD8⁺ T cells.

MHC class I-restricted CD8⁺ T cells are important in contact hypersensitivity responses in humans and mice and are the major effector cells for many allergens. For example, lipid-soluble urushiol from poison ivy enters the cytoplasm of APCs and haptened cytoplasmic proteins are processed through the MHC class I pathway, leading to the activation of allergen-specific CD8⁺ T cells. Hapten-specific CD4 T cells are also activated hapten–peptide conjugates in association with MHC class II molecules and become effector/memory CD4⁺ T cells, which contribute to the skin inflammation, or regulatory CD4⁺ T cells (Fig. 26.4).

Fig. 26.4 Sensitization phase of contact hypersensitivity

The hapten forms a hapten–carrier complex in the epidermis or within cytoplasm. Langerhans’ cells and dermal Dendritic cells internalize the antigen, undergo maturation, and migrate via afferent lymphatics to the paracortical area of the regional lymph node where peptide–MHC molecule complexes on the surface of the Langerhans’ cell can also be directly haptenated. As interdigitating cells, they present antigen to CD4⁺ and CD8⁺ T cells.

Activated T cells change the pattern of adhesion molecules on their surface by downregulating the chemokine receptor, CCR7, and CD62L.

The expression of leukocyte functional antigen-1 (LFA-1), very late antigen-4 (VLA-4), and the chemokine receptors CXCR3 and CCR5 is increased. As a result the activated/memory T cells remain within the circulation rather than trafficking through lymphoid tissue, and are able to bind to adhesion molecules on the endothelium of inflamed tissues.

Elicitation involves recruitment of CD4⁺ and CD8⁺ lymphocytes and monocytes

The application of a contact allergen leads to:

Fig. 26.5 Elicitation phase of contact hypersensitivity

Langerhans’ cells carrying the hapten–carrier complex (1) move from the epidermis to the dermis, where they present the hapten–carrier complex to memory CD4⁺ and CD8⁺ T cells (2). Activated CD4⁺ and CD8⁺ T cells release IFNγ, which induces expression of ICAM-1 (3) and, later, MHC class II molecules (4) on the surface of keratinocytes and on endothelial cells of dermal capillaries, and activates keratinocytes, which release proinflammatory cytokines such as IL-1, IL-6, and GM–CSF (5). Hapten specific CD8⁺ T cells induce apoptosis of keratinocytes expressing haptenated self-peptides (6). Non-antigen-specific T cells are attracted to the site by cytokines (7) and may bind to keratinocytes via ICAM-1 and MHC class II molecules. Activated macrophages are also attracted to the skin, but this occurs later. Thereafter the reaction starts to downregulate. This suppression is driven by eicosanoids such as prostaglandin E2 (PGE2),produced by activated keratinocytes and macrophages, and the inhibitory cytokines, IL-10 and TGFβ (8).

There is induction of mRNA for TNF, IL-1β, and GM–CSF in Langerhans’ cells within 30 minutes of exposure to allergen, and increased transcription of mRNA for IL-1α, macrophage inflammatory protein-2 (CXCL2), and interferon-induced protein-10 (CXCL10) by keratinocytes.

TNF and IL-1 are potent inducers of endothelial cell adhesion molecules, including:

Fig. 26.6 Cytokines, prostaglandins, and cellular interactions in contact hypersensitivity

Cytokines and prostaglandins are central to the complex interactions between Langerhans’ cells, CD8⁺ and CD4⁺ T cells, keratinocytes, macrophages, and endothelial cells in contact hypersensitivity. The act of antigen presentation (1) causes the release of a cascade of cytokines (2). This cascade initially results in the activation and proliferation of CD4⁺ T cells (3), the induction of expression of ICAM-1 and MHC class II molecules on keratinocytes and endothelial cells (4), and the attraction of further T cells and macrophages to the skin (3, 5). Subsequently, influx of FoxP3⁺ CD25⁺ CD4⁺ regulatory T cells inhibits T cell activation and function by direct CTLA4-mediated effects and secretion of IL-10 and TGFβ. IL-10 is also released by keratinocytes and mast cells, while keratinocytes and macrophages produce PGE, which inhibits IL-1 and IL-2 production. The combined effects of enzymatic and cellular degradation of the hapten–carrier complex, regulatory CD4⁺ T cells and suppressive cytokines and PGE released by skin cells lead to downregulation of the reaction.

VCAM-1 and ICAM-1 are the receptors for VLA-4 and LFA-1, respectively, on the surface of effector/memory T cells and contribute to their recruitment across the endothelium. These locally released cytokines and chemokines also produce a gradient signal for the movement of mononuclear cells towards the dermoepidermal junction and epidermis.

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