Immunity 24, 203-215; February 2006) The text related to Figure 2F in the Results section of our paper conveys an incorrect calculation that affects the interpretation of the experiments in Figure 2F, although not the overall conclusion we drew. The text as stated indicates that very few DCs entered efferent lymph to arrive at a lymph node (LN) further downstream. However, this extremely low magnitude (0.01%) is incorrect because it was mistakenly derived from the total fraction of LN cells, not the fraction of migrating, labeled DCs. The correct value, as is evident from the figure itself, is that 12% of traceable DCs traversed a primary draining lymph node (popliteal) to enter the downstream iliac lymph node (397 6 84 labeled DCs in the popliteal LN versus 49 6 21 in the iliac LN). This value was not affected by immunization that induced inflammation in the popliteal LN, as originally stated. Although these data indicate that DCs enter efferent lymph to a reasonable extent, the magnitude is far too low to account for the difference in the number of DCs found in LNs that primarily drain nonimmunized and immunized LNs (such as in Figure 1C).

Dendritic cell (DC) migration from the periphery to lymph nodes is regulated by the pattern of genes expressed by DCs themselves and by signals within the surrounding peripheral environment. Here, we report that DC mobilization can also be regulated by signals initiated within the downstream lymph nodes, particularly when lymph nodes enlarge as a consequence of immunization. Lymph node B lymphocytes orchestrate expansion of the lymphatic network within the immunized lymph node. This expanded network in turn supports increased DC migration from the periphery. These results reveal unique relationships between B cells, lymphatic vessels, and migratory DCs. Knowledge that DC migration from the periphery is augmented by B cell-dependent signals reveals new potential strategies to increase DC migration during vaccination.

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Purine and pyrimidine nucleotides play critical roles in DNA and RNA synthesis as well as in membrane lipid biosynthesis and protein glycosylation. They are necessary for the development and survival of mature T lymphocytes. Activation of T lymphocytes is associated with an increase of purine and pyrimidine pools. However, the question of how purine vs pyrimidine nucleotides regulate proliferation, cell cycle, and survival of primary T lymphocytes following activation has not yet been specifically addressed. This was investigated in the present study by using well-known purine (mycophenolic acid, 6-mercaptopurine) and pyrimidine (methotrexate, 5-fluorouracil) inhibitors, which are used in neoplastic diseases or as immunosuppressive agents. The effect of these inhibitors was analyzed according to their time of addition with respect to the initiation of mitogenic activation. We showed that synthesis of both purine and pyrimidine nucleotides is required for T cell proliferation. However, purine and pyrimidine nucleotides differentially regulate the cell cycle since purines control both G1 to S phase transition and progression through the S phase, whereas pyrimidines only control progression from early to intermediate S phase. Furthermore, inhibition of pyrimidine synthesis induces apoptosis whatever the time of inhibitor addition whereas inhibition of purine nucleotides induces apoptosis only when applied to already cycling T cells, suggesting that both purine and pyrimidine nucleotides are required for survival of cells committed into S phase. These findings reveal a hitherto unknown role of purine and pyrimidine de novo synthesis in regulating cell cycle progression and maintaining survival of activated T lymphocytes.