Hematopoiesis of immature myeloid dendritic cells in stroma-dependent spleen long-term cultures occurs independently of NF-KB/RelB function

MCSF drives regulatory DC development in stromal co-cultures supporting hematopoiesis

Background

Splenic stroma overlaid with hematopoietic progenitors supports in vitro hematopoiesis with production of dendritic-like cells. Co-cultures of murine lineage-depleted bone marrow over the 5G3 stromal line produce two populations of cells, characterised as CD11b⁺CD11c⁺MHC-II⁻ dendritic-like ‘L-DC’, and CD11b⁺CD11c⁺MHC-II⁺ cells, resembling conventional dendritic cells (cDC). To date, the functional capacity of these two subsets has not been clearly distinguished.

Results

Here we show both the L-DC and cDC-like subsets can be activated and induce proliferation of OT-I CD8⁺ T cells, being strong inducers of IL-2 and IFN-γ production. Both subsets lack ability to induce proliferation of OT-II CD4⁺ T cells. The cDC-like population is shown here to resemble regulatory DC in that they induce FoxP3 expression and IL-10 production in OT-II CD4⁺ T cells, in line with their function as regulatory DC. L-DC did not activate or induce the proliferation of CD4⁺ T cells and did not induce FoxP3 expression in CD4⁺ T cells. L-DC can be distinguished from cDC-like cells through their superior endocytic capacity and expression of 4-1BBL, F4/80 and Sirp-α. A comparison of gene expression by the two subsets was consistent with L-DC having an activated or immunostimulatory DC phenotype, while cDC-like cells reflect myeloid dendritic cells with inflammatory and suppressive properties, also consistent with functional characteristics as regulatory DC. When a Transwell membrane was used to prevent hematopoietic cell contact with stroma, only cDC-like cells and not L-DC were produced, and cell production was dependent on M-CSF production by stroma.

Conclusion

Co-cultures of hematopoietic progenitors over splenic stroma produce two distinct subsets of dendritic-like cells. These are here distinguished phenotypically and through gene expression differences. While both resemble DC, there are functionally distinct. L-DC activate CD8⁺ but not CD4⁺ T cells, while the cDC-like population induce regulatory T cells, so reflecting regulatory DC. The latter can be enriched through Transwell co-cultures with cell production dependent on M-CSF.

RelB: an outlier in leukocyte biology

RelB is one of the more unusual members of the NF-κB family. This family, arguably the best known group of transcription regulators, regulates an astonishing array of cell types and biological processes. This includes regulation of cell growth, differentiation and death by apoptosis, and the development and function of the innate and adaptive-immune system. RelB is best known for its roles in lymphoid development, DC biology, and noncanonical signaling. Within the last few years, however, surprising functions of RelB have emerged. The N-terminal leucine zipper motif of RelB, a motif unique among the NF-κB family, may associate with more diverse DNA sequences than other NF-κB members. RelB is capable of direct binding to the AhR that supports the xenobiotic-detoxifying pathway. RelB can regulate the circadian rhythm by directly binding to the BMAL partner of CLOCK. Finally, RelB also couples with bioenergy NAD(+) sensor SIRT1 to integrate acute inflammation with changes in metabolism and mitochondrial bioenergetics. In this review, we will explore these unique aspects of RelB, specifically with regard to its role in immunity.

Distinct In Vitro Myelopoiesis is Dependent on the Self-Renewal of Hematopoietic Progenitors

Spleen and bone marrow (BM) have been shown to contain the progenitors of a novel dendritic-like antigen-presenting cell type (L-DC). These progenitors are also maintained in both long-term spleen cultures and co-cultures of spleen or BM over the stromal cell line STX3. We examined mouse foetal liver (FL), rich in hematopoietic stem/progenitor cells (HSC/HPC) after embryonic day (E) 12.5, for the presence of L-DC progenitors by testing their capacity to colonize STX3 and produce L-DC. E14.5 FL from wild-type C57BL/6J mice was found to colonize STX3 and produce L-DC for 28 days. By contrast, E14.5 FL from Ikaros Plastic mice gave only short-term production of low numbers of L-DC between 7 and 14 days of co-culture. The transient and weak production of L-DC by FL from Plastic E14.5 mice maps to the loss of self-renewal capacity amongst HSC. L-DC progenitors are, therefore, closely aligned with a subset of self-renewing HSC/HPC in FL.

Hematopoiesis leading to a diversity of dendritic antigen-presenting cell types

Hematopoietic stem cells (HSCs) undergo expansion and differentiation, giving rise to all terminally differentiated blood cells throughout life. HSCs are found in distinct anatomical sites during development, and in adults, hematopoiesis occurs predominantly on the luminal side of the bone cavity in bone marrow. Millions of newly formed blood cells are generated per second to accommodate the short half-life of hematopoietic cells. For this to happen, HSCs must sustain their self-renewal capacity as well as their capability to commit and differentiate toward multiple cell lineages. Development of the hematopoietic system is finely regulated as the animal ages, so that it does not become exhausted or misdirected. This review covers aspects of hematopoietic development from the embryonic period through adult life in relation to development of dendritic cells. It also considers a role for HSCs in extramedullary sites and their possible role in myelopoiesis, with formation of tissue-specific antigen-presenting cells.

Investigation of murine spleen as a niche for hematopoiesis

BACKGROUND

Spleen as a lymphoid tissue is specialized for monitoring blood and mounting immunity against blood-borne antigens. Antigen-presenting cells present in spleen commonly develop from bone marrow-derived precursors that enter blood circulation. However, a distinct splenic myeloid antigen-presenting cell subset described in this laboratory, namely "dendritic-like cells" (L-DC), has been hypothesized not to share a bone marrow origin.

METHODS

In this study, the presence of endogenous hematopoietic progenitors in spleen was investigated by transplanting intact spleen into allotype-distinct recipients and monitoring development of progeny cells in grafted tissues.

RESULTS

Successful engraftment of donor spleens was achieved for up to 4 weeks. After 2 weeks, donor-type myeloid cells, dendritic cells (DC) and few B cells were observed in spleen grafts indicative of spleen-endogenous hematopoiesis. An influx of host-type hematopoietic stem cell, as well as host-type lymphoid and myeloid cells, was also observed.

CONCLUSION

Evidence for the maintenance of donor-type myeloid cells after 2 to 4 weeks reflected development from hematopoietic progenitors endogenous to spleen. L-DC were also detected in spleen grafts; however, within the graft microenvironment they were one of the several DC and myeloid subsets, including activated DC, resembling L-DC. Overall, this study adds further evidence that spleen can support endogenous myelopoiesis.