Contact between T cells and dendritic cells (DCs) is required for their subsequent interaction leading to the induction of adaptive immune responses. Quantitative data regarding the contact frequencies of T cell subsets in different lymphoid organs and species are lacking. Therefore, naive, effector, and memory CD4 T cells were injected into rats in absence of the cognate Ag, and 0.5–96 h later, spleen, lymph nodes, and Peyer’s patches were removed. Cryosections were analyzed for contact between donor T cells and endogenous DCs in the T cell zone, and donor cell proliferation. More than 60% of injected naive CD4 T cells were in contact with endogenous DCs at all time points and in all organs analyzed. Surprisingly, we were unable to detect any differences between naive, effector, and memory CD4 T cells despite different expression levels of surface molecules. In addition, contact frequency was similar for T cells in lymphoid organs of rats, mice, and humans; it was unaffected by the absence of LFA-1 (CD11a/CD18), and sustained effector T cells in an activated state. Thus, the architecture of the T cell zone rather than expression patterns of surface molecules determines the contact efficiency between T cells and DCs in vivo.

Effector T cells generated in the mesenteric lymph nodes (mLN) are known to accumulate in mLN and the tissue drained by them after circulating in the blood. Their accumulation is due less to preferential entry into mLN but more to preferential proliferation within mLN. The factors regulating the proliferation of effector T cells in vivo are unclear, and it is unknown whether they are different for CD4+ and CD8+ effector T cells. Rat T cells from mLN or peripheral lymph nodes (pLN) were stimulated polyclonally via the TCR and CD28 and injected i.v. into congenic recipients. Using three-color flow cytometry and immunohistochemistry, they were identified in mLN, pLN, and blood over time, and proliferation was determined by measuring bromodeoxyuridine incorporation. Only effector mLN T cells showed a significantly increased proliferation rate after entry into mLN compared with that in pLN (2.4 ± 1.8% vs 0.8 ± 0.4%). Proliferation among the injected cells was higher when they had contact with dendritic cells within mLN (9.0 ± 4.3%) than when they did not (4.1 ± 2.1%). Furthermore, effector mLN T cells which were observed 56 days after injection maintained the capacity for preferential proliferation within mLN. Interestingly, CD4+ effector mLN T cells proliferated at a higher rate (4.8 ± 0.7%), remaining in mLN, whereas CD8+ effector mLN T cells proliferated at a lower rate (3.3 ± 1.0%) and were able to leave the mLN into the blood. Elucidating the factors regulating the proliferation of effector T cells in vivo will help to modify their distribution for therapeutic purposes.