In the rabbit a number of large mononuclear cells with ruffled surface membranes travel from the skin and superficial tissues of the leg, via the lymphatics, to the popliteal lymph node: they constitute 40-50% of the total cell population in the afferent lymph. About 10% of these cells are actively phagocytic when tested in vitro and about 3% are found to contain Langerhans granules. After isotopic labelling the majority of lymph-borne mononuclear cells can be detected within the regional node for at least 24 hours; most being located in the paracortex and a few in the interfollicular cortex. It is proposed that these cells, including those containing Langerhans granules, belong to the "mononuclear phagocyte system." Possible functions of these lymph-borne cells are discussed with particular reference to antigen transport.

Large mononuclear cells with long, actively moving cytoplasmic veils were observed in lymph coming from the skin. The enzyme histochemistry and ultrastructure of these cells suggested that they are related to epidermal Langerhans cells and interdigitating cells in the lymph node. It has been reported that Langerhans cells and interdigitating cells play a role in contact hypersensitivity by taking up antigen and presenting it to thymus-dependent lymphocytes, and it is likely that the veiled cells in the lymph are also involved. After skin-painting with 1-fluoro-2,4-dinitrobenzene (DNFB), the veiled cells in lymph coming from the site of painting became more active and were observed contacting other cells present in the lymph; many large cellular aggregates were found. Since neutrophilic leucocytes and mononuclear phagocytes were the predominating cell types in this lymph, there was no evidence for a massive recruitment of immunocompetent lymphocytes at the site of painting. Neonatally thymectomized pigs do not develop allergic reactivity to DNFB. It is of interest that the number of veiled cells and their ability to form large cellular aggregates was not affected in these animals. Therefore, it is unlikely that the defect in responsiveness can be attributed to a failure in the function of veiled cells.

Peripheral blood lymphocytes from normal rabbits or from rabbits hyperimmunized with human immunoglobulin were cultured in 20-microliters hanging droplets. The cultures were stimulated with concanavalin A or, in the case of cells from sensitized animals, with human immunoglobulin. The addition to the cultures of small numbers of autologous or allogeneic veiled cells separated from afferent lymph increased the responses to low doses of the stimulants, particularly when the cells were cultured at low cell densities or for short periods of time. At high cell densities or at longer periods of culture, stimulation occurred in the absence of added veiled cells and was associated with clumping of the lymphoid cells and development of cells which morphologically resembled veiled cells found in afferent lymph. The enhanced responses upon addition of small numbers of veiled cells were also correlated with the formation of lymphoid aggregates which were frequently held together by the elongated processes of a single veiled cell. The observations support the view that the veiled cells are lymph-borne precursors of dendritic cells in the lymph nodes. This may provide an in vitro model for the cellular relationships which occur in paracortical cords of lymph nodes following antigenic stimulation.

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Okabayashi, T. (Shionogi Research Laboratory, Fukushimaku, Osaka, Japan). Occurrence of nucleotides in culture fluids of microorganisms. III. Nucleotide excretion by Brevibacterium liquefaciens sp. n. with special reference to uridine diphospho-N-acetylglucosamine compounds. J. Bacteriol. 84:1-8. 1962.-Among various nucleotides excreted in the culture broth of Brevibacterium liquefaciens sp. n., four uridine nucleotides containing N-acetylglucosamine were isolated. These were found to be uridine diphospho-N-acetylglucosamine and Park's compounds 1, 2, and 3. The significance of the appearance of these cell-wall precursors in the culture fluid of B. liquefaciens is discussed.