Jürgen Westermann

Although the liver is known to contain a significant number of lymphocytes, migration of these through the compartments of the liver, parenchyma and periportal field, has not been studied. The periportal field, in particular, is affected in several immunological disorders of the liver. Populations of labeled naive, activated, and memory T cells were injected into congenic rats. The recipient livers and draining lymph nodes were removed at various time points, and cryostat sections were analyzed for the presence of donor cells using quantitative immunohistology. Donor cell proliferation and apoptosis were examined in vivo by BrdU (5 μM 5-bromo-2-deoxyuridine) incorporation and the TUNEL technique, respectively. Early after injection (0.5–1 h), naive, activated, and memory T cells were localized to the parenchyma and periportal field in comparable numbers. With time, all T cell subsets left the parenchyma but remained or, in the case of activated T cells, significantly accumulated in the periportal field. Furthermore, 12% of activated donor T cells proliferated in vivo within the periportal field, and 0.5% showed evidence of apoptosis. Taken together, not only activated and memory, but also naive T cells continuously migrate through the liver, showing a preference for the periportal field, and activated T cells mainly proliferate there. This may explain why many immunological liver diseases predominantly affect the periportal field.

It is thought that naive T cells predominantly enter lymphoid organs such as lymph nodes (LN) and Peyer's patches (PP) via high endothelial venules (HEV), whereas memory T cells migrate mainly into non‐lymphoid organs. However, direct evidence for the existence of these distinct migration pathways in vivo is incomplete, and nothing is known about their migration through the different compartments of lymphoid organs. Such knowledge would be of considerable interest for understanding T cell memory in vivo. In the present study we separated naive and memory CD4⁺ T cells from the rat thoracic duct according to the expression of the high and low molecular weight isoforms of CD45R, respectively. At various time points after injection into congenic animals, these cells were identified by quantitative immunohistology in HEV, and T and B cell areas of different LN and PP. Three major findings emerged. First, both naive and memory CD4⁺ T cells enter lymphoid organs via the HEV in comparable numbers. Second, naive and memory CD4⁺ T cells migrate into the B cell area, although in small numbers and continuously enter established germinal centers (GC) with a bias for memory CD4⁺ T cells. Third, memory CD4⁺ T cells migrate faster through the T cell area of lymphoid organs than naive CD4⁺ T cells. Thus, our study shows that memory CD4⁺ T cells are not excluded from the HEV route. In addition, “memory” might depend in part on the ability of T cells to specifically enter the B cell area and GC and to screen large quantities of lymphoid tissues in a short time.

After activation within a lymphoid tissue, T lymphocytes enter the blood, where they circulate and then re‐enter many organs. However, they predominantly end up in the tissue of origin, a phenomenon so far thought to be caused by organ‐specific homing. We analyzed the fate of T cells from different sources stimulated via the T cell receptor and CD28 and then injected intravenously into rats. Our results showed that preferential proliferation and reduced apoptosis, rather than preferential immigration, were responsible for the accumulation of activated T cells in the tissue of origin, explaining how immune responses can spread from site to site but still be restricted to certain regions. Manipulating the life span of such cells might be a promising approach to influencing immune responses.

Although the thymus is primarily noted for the export of T cells to the periphery, a small influx of cells has also been observed. It is still a matter of debate whether entry into the thymus depends on prior activation. The phenotypes, sources and degree of immigration are largely unknown. We monitored by quantitative immunohistochemistry the entry of cells from the periphery into the rat thymus in three experimental models. We injected i.v. recirculating, small, nonactivated CD4⁺ T cell subsets, often referred to as naive (CD45RC⁺) and memory or antigen‐experienced (CD45RC⁻) cells, purified from thoracic duct lymph of allotype‐marked donors, allotype‐marked leukocytes released from spleen or lung transplants, or leukocytes labeled in the periphery for 12 weeks during the S‐phase of the cell cycle by oral application of 5‐bromo‐2‐deoxyuridine (BrdUrd). Early after i.v. injection (0.5 h), significantly more antigen‐experienced (CD45RC⁻) CD4⁺ T cells entered the thymus, and by 24 h four times as many cells from the CD45RC⁻ subset as from the CD45RC⁺ subset had entered the thymus and localized to the medulla. None of the thymic entrants expressed the interleukin (IL)‐2 receptor. Following spleen transplantation ∼ 40% of donor cells entering the thymic medulla were T cells and ∼ 55% were B cells. In contrast, from a lung transplant, ∼ 85% of peripheral immigrants were T cells and ∼ 10% were B cells. After both procedures, a small number of NK cells and monocytes/macrophages were found among the immigrants (< 5%). Rats were fed BrdUrd continuously for 12 weeks, a procedure which labeled ∼ 30% of peripheral lymphocytes but not cortical thymocytes. BrdUrd‐labeled cells were localized almost exclusively to the thymic medulla and represented ∼ 10% of medullary cells. Of the thymic immigrants ∼ 50% were T cells, ∼ 30% were B cells (including ∼ 15% IgD⁺ cells), ∼ 15% were NK cells and the remainder (∼ 5%) were monocytes/macrophages. Only a quarter of BrdUrd‐labeled cells expressed the IL‐2 receptor. The thymus is continuously infiltrated by both activated and nonactivated leukocytes from the periphery, including T cells, B cells, NK cells and monocytes. These immigrants are supplied by lymphoid and nonlymphoid organs in a characteristic subset composition. Their entry is facilitated by prior antigen experience or activation. Thus, the participation of the thymic medulla in general leukocyte traffic suggests a mechanism by which the T cell repertoire could potentially be modulated by the peripheral tissues.