CD4(+) T lymphocytes are key to immunological memory. Here we show that in the memory phase of specific immune responses, most of the memory CD4(+) T lymphocytes had relocated into the bone marrow (BM) within 3-8 weeks after their generation-a process involving integrin alpha2. Antigen-specific memory CD4(+) T lymphocytes highly expressed Ly-6C, unlike most splenic CD44(hi)CD62L(-) CD4(+) T lymphocytes. In adult mice, more than 80% of Ly-6C(hi)CD44(hi)CD62L(-) memory CD4(+) T lymphocytes were in the BM. In the BM, they associated to IL-7-expressing VCAM-1(+) stroma cells. Gene expression and proliferation were downregulated, indicating a resting state. Upon challenge with antigen, they rapidly expressed cytokines and CD154 and efficiently induced the production of high-affinity antibodies by B lymphocytes. Thus, in the memory phase of immunity, memory helper T cells are maintained in BM as resting but highly reactive cells in survival niches defined by IL-7-expressing stroma cells.

In the bone marrow (BM), memory plasma cells (PCs) survive for long time periods in dedicated microenvironmental survival niches, resting in terms of proliferation. Several cell types, such as eosinophils and reticular stromal cells, have been reported to contribute to the survival niche of memory PCs. However, until now it has not been demonstrated whether the niche is formed by a fixed cellular microenvironment. By intravital microscopy, we provide for the first time evidence that the direct contacts formed between PCs and reticular stromal cells are stable in vivo, and thus the PCs are sessile in their niches. The majority (ß80%) of PCs directly contact reticular stromal cells in a non-random fashion. The mesenchymal reticular stromal cells in contact with memory PCs are not proliferating. On the other hand, we show here that eosinophils in the vicinity of longlived PCs are vigorously proliferating cells and represent a dynamic component of the survival niche. In contrast, if eosinophils are depleted by irradiation, newly generated eosinophils localize in the vicinity of radiation-resistant PCs and the stromal cells. These results suggest that memory PC niches may provide attraction for eosinophils to maintain stability with fluctuating yet essential accessory cells. Keywords: Bone marrow r Intravital microscopy r Plasma cells r Stromal cells r Survival niche See accompanying Commentary by Tellier and KalliesAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionThe prominent role of the bone marrow (BM) for the maintenance of resting immunological memory is becoming increasingly evident. Several types of memory lymphocytes have been identified which are maintained predominantly in the BM [1]. Longlived, resting memory plasma cells (PCs) reside in the BM [2] and resting professional memory CD4 + T cells have been located to the BM as well [3]. The BM is thought to sustain the survival of Correspondence: Prof. Anja E. Hauser e-mail: hauser@drfz.de these different memory immune cell populations in cell-specific microanatomical niches [4,5]. PCs have been shown to depend on extrinsic survival factors to survive and their colocalization with several cell types which produce these survival factors has been demonstrated by histology. They are in close contact with reticular stromal cells producing chemokine (CXC motif) ligand 12 (CXCL12) [6], a chemokine which has been shown to regulate PC immigration into the BM [7,8], but which also acts as a potent PC survival factor in vitro [9]. Blocking of leukocyte * These authors contributed equally to this work. Results Plasma cells colocalize with stromal cells in the bone marrowPCs of the BM have been described to contact reticular stromal cells, but this has not been quantified so far, because of the difficulty to identify contacts between PCs and dendritic extensions of stromal cells. Here we used genetic staining of stromal cells to better visualize their ramifications and potential contacts to PCs...

It is believed that memory CD8+ T cells are maintained in secondary lymphoid tissues, peripheral tissues, and BM by homeostatic proliferation. Their survival has been shown to be dependent on IL-7, but it is unclear where they acquire it. Here we show that in murine BM, memory CD8+ T cells individually colocalize with IL-7+ reticular stromal cells. The T cells are resting in terms of global transcription and do not express markers of activation, for example, 4-1BB (CD137), IL-2, or IFN-γ, despite the expression of CD69 on about 30% of the cells. Ninety-five percent of the memory CD8+ T cells in BM are in G0 phase of cell cycle and do not express Ki-67. Less than 1% is in S/M/G2 of cell cycle, according to propidium iodide staining. While previous publications have estimated the extent of proliferation of CD8+ memory T cells on the basis of BrdU incorporation, we show here that BrdU itself induces proliferation of CD8+ memory T cells. Taken together, the present results suggest that CD8+ memory T cells are maintained as resting cells in the BM in dedicated niches with their survival conditional on IL-7 receptor signaling.