Dendritic cells (DC) were not on my mind until I saw one in an "Under Agarose" chemotaxis assay I was doing in 1979. I was quite amazed to see a DC because antigen presenting cells were presumably not present in thoracic duct lymph (TDL). The immunological importance of finding dendritic cells in TDL struck me because Ralph Steinman had shown in 1978 that dendritic cells were more potent antigen presenting cells in vitro than any other known accessory cells. DC in TDL made it conceivable that DC precursors could transport antigenic "memory" to other tissues.

To analyze whether or not this was induced by an inflammatory stimulus, Jon Warren and I collected TDL from normal and adjuvant-treated Lewis rats. DC "precursors" were 0.025% of normal TDL, and DC output increased many hundred fold after adjuvant inoculation. A billion cells return to the blood in TDL each day, so 0.025-1.5% of TDL is a lot of DC. G.G. MacPherson and colleagues found "veiled cells" in intestinal lymph after MLN excision, and E.B. Bell found antigen-laden small monocytes in TDL, but we were the first to show that DC are normally among the cells returning to the blood in efferent lymph. We also linked changes in output of DC in TDL with effects of adjuvants.

When TDL are incubated in a T-25 flask (far left), small monocytes (m) and dendritic cells (dc) differentiate over 48 - 96 hours incubation. Dendritic Cell differentiation is complete by 96 hours. This enabled us to enumerate DC and M "precursors" in TDL, and to study whether the cells were phagocytic, expressed Class II MHC, Fc and Complement Receptors and other markers for T or B cells. These studies concluded that these cells were identical to those described by Ralph Steinman. We also found that there were two types of DC present. One type expressed receptors for C3b, stimulated lymphocyte division and enhanced IgA>>IgG immunoglobulin secretion by B cells. The other DC type, lacked C3b receptor but stimulated division of T cells and induced alloreactive cytotoxicity. A Science article also described two types of DC. Just one of them made IFN needed for (Th1) CTL commitment. It is not known if the other favors Th2 development.

Because DC isolated from thoracic duct lymph were "undifferentiated," their longevity in vitro was superior to that described for DC isolated by disaggregating lymphoid tissues and separation by centrifugation onto a 30% albumen cushion, and they remained anchored to the substratum beyond 48 hours. These dendritic cells remained viable through 5 weeks of culture when rare fibroblastic cells present in the inoculum overgrew the plate. We also found that aggregates of dividing cells would form around DC when syngeneic lymphocytes were added back to the cultures of DC and M.

References for Art's early "image-directed" observations are among those listed below.