Dendritic Cells

Dendritic cells (DCs) get their name from their surface projections (that resemble the dendrites of neurons — see figure).

They are found in most tissues of the body and are particularly abundant in those that are interfaces between the external and internal environments, e.g., skin and the lining of the gastrointestinal tract, where they are ideally placed to encounter invading pathogens.

Two dendritic cells (arrows) from the spleen of a mouse. Compare their smooth surface with that of the two macrophages visible at the upper left. (Courtesy of Ralph Steinman, from R. M. Steinman et al., J. Exp. Med. 149:1, 1979.)

• They are actively motile.
• They continuously sample their surroundings — ingesting antigens by endocytosis (using phagocytosis, receptor-mediated endocytosis, and pinocytosis).
  • Many of these antigens are "self" antigens, e.g., dead cells, proteins in the extracellular fluid.
  • But the antigens can also be foreign antigens, for example, bacteria that are resident in the body (e.g., in the colon) or that invade the body.

  In either case, the ingested antigens are degraded in lysosomes into peptide fragments that are then displayed at the cell-surface nestled in class II MHC molecules [Link].

Having ingested antigen in the tissue, they migrate to lymph nodes and spleen where they can meet up with T cells bearing the appropriate T-cell receptor for antigen (TCR).

• Self antigens are presented to T cells without any costimulatory molecules. This interaction causes the T cells to divide for a brief time, but then they commit suicide by apoptosis and so cannot attack tissues of the body. The animal becomes tolerant to that antigen.
• Foreign antigens produce a different outcome. The dendritic cells becomes "activated' and begin to display not only
  • the MHC-peptide complex for the TCR of the T cells [Link] but also
  • costimulatory molecules, e.g. B7 which binds to CD28 on the T cell. [Link]

What accounts for the activation of dendritic cells by foreign antigens but not by self antigens?

• are not shared with their host;
• are shared by many related pathogens;
• are relatively invariant; that is, do not evolve rapidly (in contrast, for example, to such pathogen molecules as the hemagglutinin and neuraminidase of influenza viruses).

Examples:

• the flagellin of bacterial flagella;
• the peptidoglycan of Gram-positive bacteria;
• the lipopolysaccharide (LPS, also called endotoxin) of Gram-negative bacteria;
• double-stranded RNA. (Some viruses of both plants and animals have a genome of dsRNA. And many other viruses of both plants and animals have an RNA genome that in the host cell is briefly converted into dsRNA [link to examples]).
• unmethylated DNA (eukaryotes have many times more cytosines, in the dinucleotide CpG, with methyl groups attached).

Dendritic cells have a set of transmembrane receptors that recognize different types of PAMPs. These are called Toll-like receptors (TLRs) because of their homology to receptors first discovered and named in Drosophila.

TLRs identify the nature of the pathogen and turn on an effector response appropriate for dealing with it. These signaling cascades lead to the expression of various cytokine genes.

• Interleukin 12 (IL-12) drives the nearby T cells to become Th1 cells, which will provide help for cell-mediated immunity including attack against intracellular pathogens.
• IL-23, which promotes differentiation of the T cells into a Th17 helper cell, which can deal with extracellular bacteria.
• IL-4, which promotes differentiation of the T cells into Th2 cells which provide help for antibody production by B cells.

Under other circumstances, activated dendritic cells may secrete TGF-β and IL-10, leading to the formation of regulatory T cells (Treg and Tr1) that dampen immune responses.

Dendritic cell subsets

While all DCs share certain features, they actually represent a variety of cell types with different differention histories, phenotypic traits and, as outlined above, different effector functions.

Examples:

Plasmacytoid Dendritic Cells

These cells get their name from their extensive endoplasmic reticulum which resembles that of plasma cells. However, unlike plasma cells that are machines for pumping out antibodies, these dendritic cells produce huge amounts of interferons (alpha and beta). They do so in response to viral infections.

• TLR-7 and TLR-8, which bind to the single-stranded RNA (ssRNA) genomes of such viruses as influenza, measles, and mumps.
• TLR-9, which binds to the unmethylated cytosines in the dinucleotide CpG in the DNA of the pathogen. (The cytosines in the host's CpG dinucleotides often have methyl groups attached.)

CD8⁺ vs. CD8⁻ Dendritic Cells

• The CD8⁻ subset presents antigen engulfed from the surroundings — using the class II pathway — to CD4⁺ helper T cells.
• However, the CD8⁺ subset is also able to present extracellular antigens using the class I pathway. The peptide/MHC class I molecules are presented to CD8⁺ T cells which go on to become cytotoxic T lymphocytes (CTL). This phenomenon is called cross-presentation [More].

Dendritic cells can also present undegraded antigen to B cells; that is, antigen that has not been processed into peptide/MHC complexes — Link.

Phase contrast micrograph of spleen cells after 2 days in culture. Four dendritic cells (arrows) can be seen clustered with lymphocytes. (Courtesy of Ralph Steinman from K. Inaba et al., J. Exp. Med. 160:858, 1984.)

Ralph Steinman, the pioneer in the study of dendritic cells, has provided striking visual evidence of the cellular interactions between antigen-presenting dendritic cells, T cells, and B cells. When spleen cells are cultured with antigen, tight clusters of cells form (see figure). The clustering occurs in two phases:

• an early phase (days 0–2) during which only the dendritic cells and T cells need to be present to form clusters;
• a later phase (days 2–5) when antigen-primed B cells enter the cluster and differentiate into antibody-secreting cells.

Homing

Some dendritic cells not only activate T cells to respond to a particular antigen but tell them where to go to deal with that antigen.

Two examples:

Antigens in the skin

• Dendritic cells engulf antigens in (or even on!*) the skin and, while doing so, convert calciferol (vitamin D₃) present in the skin into calcitriol (1,25[OH]₂ Vitamin D₃) [Link].
• When they activate the appropriate T cells in a nearby lymph node, the calcitriol induces those T cells to express a surface receptor designated CCR10 (a member of the CC chemokine receptor family).
• CCR10 binds the chemokine CCL27 — which is present in the skin.
• So when these T cells reach the skin, they stop their travels and go to work there.

Antigens in the GI tract

• Dendritic cells in the lining of the intestine are always busy engulfing the many antigens present there.
• While doing so, they convert the abundant amount of retinol (vitamin A) there into retinoic acid.
• When they activate the appropriate T cells in a nearby lymph node, the retinoic acid induces those T cells to express another CC chemokine receptor designated CCR9.
• CCR9 binds the chemokine CCL25 present in the intestine.
• So when these T cells reach the intestine, they stop their travels and go to work. (CCL25 also attracts IgA-secreting B cells.)

(In telling this story, I cannot help being reminded of the way that scout bees, having found food, return to the hive and tell the worker bees there where to go to find it! [Link])