Nonlymphoid dendritic cells

DCs in nonlymphoid tissues perform a sentinel function in acquiring foreign antigens before maturing and migrating to secondary lymphoid tissues for initiation of many immune responses. Hence, in general, the cells in situ represent the immature or processing stage of the lineage.

1) DCs are present within skin and the epithelia of the respiratory, gastrointestinal and urogenital tracts. The DCs of skin, Langerhans cells (LCs), form a ramifying network throughout the epidermis, together, in mouse but not human, with epidermal y8 T cells whose functions have yet to be ascertained. It is estimated that the half-life of LCs in unperturbed skin epidermis is likely to range from weeks to even a month or two. Precursors to LCs are probably-present within the dermis of skin, in which a population of so-called dermal dendrocytes has been identified. Similar cells are distributed throughout the lung and airways, but are juxtaposed to macrophages which appear to regulate DC function by maintaining the transiently resident DCs at an immature stage. In gut, the lamina propria contains large numbers of DCs. In rats, following oral feeding or intraintcstinal injection of antigen, and after aerosol administration of antigens to the lung and airways, DCs from these sites can present the antigen to sensitized T cells in vitro and can prime naive recipients after adoptive transfer in vivo. 2) DCs are present in the interstitial spaces of all solid organs, with the exceptions of the bulk of the ccntral nervous system and some other sites considered to be 'immunologically privileged' such as the testis. Interstitial DCs probably acquire antigens that gain access to these sites. The half-life of interstitial DCs, such as those in heart, may range from a few days to a week or so. 3) The liver may represent a special case, in that DC progenitors may be recruited to the liver sinusoids, develop into the processing stage, and acquire particulate and perhaps other antigens that enter the bloodstream.

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