Static and dynamic components synergize to form a stable survival niche for bone marrow plasma cells

Abstract

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...

Immunological memories of the bone marrow

“…Likewise, as discussed above, BCMA can be activated by either April or BAFF 36. VCAM‐1 and ICAM‐1, the ligands of VLA‐4 and LFA‐1, are expressed by bone marrow stromal cells40 and, expectedly, in the bone marrow most if not all plasma cells contact stromal cells expressing VCAM‐1 41, 42. Mesenchymal stromal cells thus qualify as organizers of a survival niche for plasma cells, probably by providing essential survival signals, namely integrin‐mediated cell contact.…”

“…First, it is obvious from histological inspection that in the bone marrow, plasma cells are not clustered but dispersed individually throughout the parenchyme 42. Apparently 1 stromal cell cannot host more than 1 memory plasma cell.…”

Automated Quantification of Hematopoietic Cell – Stromal Cell Interactions in Histological Images of Undecalcified Bone

“…3. The next day, reconstitute mice by an intravenous injection of 3 x 10 6 bone marrow cells prepared from long bones of C57BL/6 donor mice in transfer buffer 9 . Keep the mice on Neomycin and vitamins for up to 2 weeks and monitor their well-being and weight during this time.…”

“…This needs to be taken into account if data from such chimeric mice is analyzed. Even though there is also a risk of hematopoietic cells of recipient origin surviving the irradiation, these cells typically only cover a very small area per field of view compared to the donor CD45 + cells, thus not affecting the histological analysis 9 . Clearly, this method should be combined in a subsequent step with a more specific detection of stromal (sub)populations.…”

“…This allows assertions to be made about the juxtaposition of various cell subsets; however, it does not give any information about putative molecular mechanisms involved in cellular interactions. Additionally, other methods such as 2-photon intravital microscopy are needed to improve the characterization of these intercellular contacts 9 . Förster-resonant energy transfer (FRET)-based systems can help to confirm if these interactions are actually functional 26 .…”