Dendritic cells

DC were originally isolated from mouse spleen. They represented a small proportion of the nucleated cells present, and were characterised by a relatively abundant cytoplasm, but arranged in extensive and highly motile dendrites rather than around the nucleus, hence their name (Steinman & Cohn, 1973). These DC were short lived and did not proliferate in culture (Steinman & Cohn, 1974).

The exact origin of the cells was unclear, although they appeared to be derived from the bone marrow (Steinman et al, 1974). The high turnover rate in vivo suggested influx from other sites (Steinman et al, 1974). They displayed high levels of MHC class II, but their function was still not apparent.

The observation that they were much more efficacious than other cells, such as B cells and macrophages, at stimulating a mixed leukocyte reaction, MLR (Klinkert et al,

1980, Nussenzweig & Steinman, 1980, Steinman & Witmer, 1978), and a syngeneic MLR (Nussenzweig & Steinman, 1980), indicated their most important function as APC. Unlike

B cell and m acrophages whose primary functions are to produce antibodies and phagocytose respectively, and who present antigen as a secondary event, the principle role o f DC is to capture antigen and present it to T cells (review ed in (B anchereau & Steinm an, 1998, Steinm an, 1991).

1.7.1- O n togeny o f D C

DC are distributed throughout the different organs o f the body. Irrespective o f site they have typical characteristics in com m on, including non-adherence to plastic, low levels o f Fc receptor and high levels o f M HC class II. In contrast, m acrophages are adherent, have high levels o f Fc receptor and variable levels o f class II. H ow ever all cells that are considered to be DC are not phenotypically identical: for exam ple, Langerhans cells (LC) found in the epiderm is have dendritic cell m orphology but also have Fc receptors. There are also other variants such as interdigitating cells in the thym us, and DC that are found in the blood.

W hile DC m ay have different nam es and som ew hat different cell surface phenotypes (e.g. LC are CD la negative and blood DC are CD la positive), m ost do share sim ilar APC properties. Those that are distributed outside the lym ph nodes all share the property o f m igration via the lym phatic system , or the blood, to lym ph nodes, and som e form o f m aturation occurs in doing so. As they mature they becom e a more hom ogeneous population (Schuler & Steinm an, 1985). W hile this is not p roof o f a com m on heritage it is suggestive, and it has been proposed that they are part o f a connected system (Steinm an, 1991). Part o f the problem lies in the fact that w hile DC in the spleen and lym ph originate from a proliferating pool o f precursors and undergo rapid turnover (Steinm an et a i, 1974), the turnover reflects m igration and recruitm ent rather than local cell division.

DC can be generated from the bone m arrow o f both mice and hum ans, albeit by slightly different methods. Both species require the cytokine GM-CSF, suggesting a myeloid lineage (Inaba et a i, 1992, Young et a i, 1995). In the m ouse G M -C SF stim ulates m ouse bone m arrow cells to becom e either granulocytes, m acrophages, or DC suggesting a com m on lineage (Inaba 1993a, Inaba e ra /., 1992). In the hum an, CD34+ bone marrow progenitors can be stim ulated to produce DC by G M -C SF and T N F -a. The addition of c-

A/f-ligand expands this population. This suggests that the converse may be true, that DC are form ed from specific DC colony-form ing units, CFU -D C , as opposed to granulocyte- m onocyte colony-form ing units, C FU -G M (Young et a i, 1995). In vitro, hum an DC can be prepared from incubating peripheral blood m onocytes in the presence o f G M -C SF and IL-4, which suppresses m onocyte outgrow th (Sallusto & Lanzavecchia, 1994), infeWi-ng a com m on precursor. Furtherm ore, as well as DC, these peripheral blood m onocytes m ature into m acrophages in the absence of the added cytokines, or with added M -C SF (Palucka et a i , 1998). If the G M -C S F and IL-4 are rem oved from the D C , they can revert to m acrophages, or die. Likewise the addition o f G M -C SF and IL-4 to m acrophages that are

not fully m atured leads to conversion into DC, as judged by m orphology, cell surface phenotype, and ability to stim ulate an M LR (Palucka et al., 1998) again suggesting a com m on lineage. It is not however clear w hether this in vitro m onocyte to DC change is relevant in vivo.

These findings suggest the follow ing hypothesis. All DC types evolve from a single bone m arrow precursor, be it CFU -D C or CFU -G M . From there they m igrate to organs and diverge som ew hat from each other. On subsequent m igration from the organ via the lym phatics to the lym phoid organs they m ature and converge to a m ature DC, w hich is evidenced by the fact that all DC in lym ph nodes appear the same, even though they m ay have com e from different tissues. In contrast to this view itiis necessary to note that there is evidence that som e DC can evolve from a non-m yeloid origin (G rouard et al., 1997, Saunders et a i, 1996). These DC originate from the so called plasm acytoid T cells o f the thym us. These do not require GM -CSF, but instead require IL-3 and IL-7 to m ature. The precise origin o f these cells is not clear, as despite their nam e they express no specific m arkers suggestive o f cells o f either lym phoid origin or o fm y e lo id origin (G rouard et al.,

1997).

1.7.2- D C m igration

DC have been found to reside in non-lym phoid tissue, where they can be exposed to a wide range o f antigens, but to function as APC they m ust endocyto.se the antigen, process it, and present it to T cells. Presentation to T cells is best perform ed in the lymph nodes and spleen, and thus m igration of DC is a necessary step. This has lead to DC in the periphery being referred to as im m une ‘sentinels’ (Ibrahim et al., 1995).

These peripheral DC, w hich include LC, are referred to as im m ature. They are characterised by the fact that they are good at processing antigen, but m ake poor accessory cells (Romani et a i, 1989). Uptake of antigen is via constitutive m acropinocytosis (Sallusto

et a i, 1995), and by phagocytosis of both apoptotic bodies (Albert et al., 1998), and live bacteria (Inaba ri/., 1993b, M atsuno cr «/., 1996, Reis e Sousa gf <3/., 1993, Svensson et al., 1997). In contrast m acrophages can phagocytose opsonised sheep red blood cells, w hich D C cannot (Inaba et al., 1993b, Reis e Sousa et al., 1993), but are unable to take up apoptotic bodies (A lbert etal., 1998). Capture of antigen by DC, especially live bacteria is probably m ediated at least partially by m annose receptor (Sallusto et a i, 1995). Finally, they have been shown to produce large am ounts o f M H C class II and invariant chain with a high cell surface turnover (Kam pgen et a i, 1991, Pure et al., 1990)

O nce antigen has been captured the DC m ust m igrate from the periphery to the lym ph nodes and spleen. M igration occurs principally to the draining lym ph nodes via the lym phatics (M atsuno et al., 1996, W einlich et a l, 1998). These m igrating DC hom e to the paracortical region o f the lymph nodes, where they interact with antigen specific T cells (Ingulli et a i, 1997).

Many substances induce migration in vivo. Systemic administration of bacteria and lipo-polysaccharide (LPS), a component of bacterial cells wall, results in a significant depletion of interstitial and epithelial DC (Roake et ai, 1995). It is not clear whether this is a direct on indirect effect. LPS also leads to the migration of DC to the T-cell areas of the lymph nodes and spleen (De Smedt et ai, 1996, Reis e Sousa et al., 1997). This migration to the T-cell areas requires help from T cells (Austyn et al., 1988). Once in this area DC are lost after 48 hours (De Smedt et al., 1996).

Induction of migration of LC can be induced by contact sensitisers and allergens (van Wilsem etal., 1994, Weinlich etal, 1998). This migration occurs very rapidly, although not all LC migrate suggesting other mechanisms (van Wilsem etal, 1994). The LC migrate from the epidermis to the dermis, where they can be observed as cords of cells moving through the dermal lymphatics (Larsen et al, 1990). Dermal DC migrate from the dermis in a similar fashion (Weinlich et al, 1998).