Recent results suggest that plasma cell longevity is not an intrinsic capacity, but depends on yet unknown factors produced in their environment. In this study, we show that the cytokines IL-5, IL-6, TNF-α, and stromal cell-derived factor-1α as well as signaling via CD44 support the survival of isolated bone marrow plasma cells. The cytokines IL-7 and stem cell factor, crucially important for early B cell development, do not mediate plasma cell survival, indicating that plasma cells and early B cells have different survival requirements. As shown in IL-6-deficient mice, IL-6 is required for a normal induction, but not for the maintenance of plasma cell responses in vivo, indicating that the effects of individual survival factors are redundant. Optimal survival of isolated plasma cells requires stimulation by a combination of factors acting synergistically. These results strongly support the concept that plasma cell survival depends on niches in which a combination of specific signals, including IL-5, IL-6, stromal cell-derived factor-1α, TNF-α, and ligands for CD44, provides an environment required to mediate plasma cell longevity.

Long-lived plasma cells in the bone marrow produce memory antibodies that provide immune protection persisting for decades after infection or vaccination but can also contribute to autoimmune and allergic diseases. However, the composition of the microenvironmental niches that are important for the generation and maintenance of these cells is only poorly understood. Here, we demonstrate that, within the bone marrow, plasma cells interact with the platelet precursors (megakaryocytes), which produce the prominent plasma cell survival factors APRIL (a proliferation-inducing ligand) and IL-6 (interleukin-6). Accordingly, reduced numbers of immature and mature plasma cells are found in the bone marrow of mice deficient for the thrombopoietin receptor ( IntroductionAntibody-secreting plasma cells are found in many tissues. However, the plasma cells that provide antigen-specific systemic antibodies for up to decades after immunization or infection predominantly reside in the bone marrow. [1][2][3] There are multiple lines of evidence that individual plasma cells can survive in humans and mice for many months at the least. [4][5][6][7] These long-lived plasma cells are important for maintaining protective antibody memory. However, autoantibody-secreting long-lived plasma cells are refractory to conventional immunosuppressive therapy and therefore represent a therapeutic challenge in autoimmune diseases. [8][9][10] Plasma cell survival is not cell-autonomous but depends on signals provided by their environment. The most potent plasma cell survival factors identified so far are a proliferation-inducing ligand (APRIL), interleukin-6 (IL-6), tumor necrosis factor-␣ (TNF-␣), stromal-derived factor-1␣, and signals transduced via CD44. [11][12][13][14] The bone marrow contains multiple microenvironmental niches that stimulate cellular proliferation, differentiation, and survival. [15][16][17][18][19][20] Each niche seems to support specifically one or a few particular hematopoietic stem or precursor cells. In this way, the sizes of these populations are limited by the number of available niches. 16,21 Similarly, competition for a limited number of survival niches may also control the turnover rate within the bone marrow plasma cell compartment. 12,[22][23][24] One or multiple niches may exist that have the capability to support the terminal differentiation and survival of bone marrow plasma cells. 25 As indicated by strong colocalization between a particular subtype of stromal-derived factor-1␣ ϩ reticular stromal cells and immunoglobulin G ϩ (IgG ϩ ) bone marrow plasma cells, the former seems to be an important element of plasma cell niches in that tissue. 26 However, in culture, bone marrow stromal cells support plasma cell survival only for a limited time, 13 suggesting that additional cell types contribute to the formation of plasma cell niches.In addition, it has been shown that macrophage-derived APRIL is required to support differentiation/survival of bone marrow plasma cells during early life, suggesting that factors ...

Key Points• Donor T-cell infiltration of the bone marrow is associated with impaired B-cell immunity after allogeneic HSCT.• Quantification of k-deleting recombination excision circles as a biomarker for bone marrow B-cell output in different clinical episodes.B-cell immune dysfunction contributes to the risk of severe infections after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Delayed B-cell regeneration is found in patients with systemic graft-versus-host disease (GVHD) and is often accompanied by bone marrow (BM) suppression. Little is known about human BM GVHD. We analyzed the reconstitution kinetics of B-cell subsets in adult leukemic patients within 6 months after allo-HSCT. B-cell deficiency already existed before transplant and was aggravated after transplant. Onset of B-cell reconstitution characterized by transitional B-cell recovery occurred either early (months 2-3) or late (from month 6 on) and correlated highly positively with reverse transcription-polymerase chain reaction quantified numbers of k-deleting recombination excision circles (KRECs). Delayed recovery was associated with systemic acute GVHD and full-intensity conditioning therapy. Histological analysis of BM trephines revealed increased T-cell infiltration in late recovering patients, which was associated with reduced numbers of osteoblasts. Functionally, late recovering patients displayed less pneumococcal polysaccharide-specific immunoglobin M-producing B cells on ex vivo B-cell activation than early recovering patients. Our results provide evidence for acute BM GVHD in allo-HSCT patients with infiltrating donor T cells and osteoblast destruction. This is associated with delayed B-cell reconstitution and impaired antibody response. Herein, KREC appears suitable to monitor BM B-cell output after transplant. (Blood. 2014;124(6):963-972)

BackgroundAfter hematopoietic stem cell transplantation (HSCT) T- and B-cell reconstitution from primary lymphoid organs are a prerequisite for an effective early lymphocyte reconstitution and a long-term survival for adult patients suffering from acute leukemia. Here, we asked whether quantification of T cell receptor excision circle, (TREC) and kappa-deleting recombination excision circle (KREC) before and within six month after allogeneic HSCT could be used to measure the thymic and bone marrow outputs in such patients.MethodsWe used a duplex real time PCR assay to quantify the absolute copy counts of TREC and KREC, and correlated the data with absolute cell counts of CD3+CD4+ T-cell and CD19+ B-cell subsets determined by flow cytometry, respectively.ResultsBy comparing two recently proposed naïve T cell subsets, CD31+ naive and CD31- naive T cells, we found a better correlation for the CD31+ subset with TREC level post alloHSCT, in line with the assumption that it contained T cells recently derived from the thymus, indicating that TREC levels reflected real thymic de novo production. Transitional as well as naïve B cells highly correlated with KREC levels, which suggested an association of KREC levels with ongoing bone marrow B cell output. CD45RO+ memory T cells and CD27+ memory B cells were significantly less correlated with TREC and KREC recovery, respectively.ConclusionWe conclude that simultaneous TREC/ KREC quantification is as a suitable and practicable method to monitor thymic and bone marrow output post alloHSCT in adult patients diagnosed with acute leukemia.

The bone marrow is the origin of all hematopoietic lineages and an important homing site for memory cells of the adaptive immune system. It has recently emerged as a graft-versus-host disease (GvHD) target organ after allogeneic stem cell transplantation (alloHSCT), marked by depletion of both hematopoietic progenitors and niche-forming cells. Serious effects on the restoration of hematopoietic function and immunological memory are common, especially in patients after myeloablative conditioning therapy. Cytopenia and durable immunodeficiency caused by the depletion of hematopoietic progenitors and destruction of bone marrow niches negatively influence the outcome of alloHSCT. The complex balance between immunosuppressive and cell-depleting treatments, GvHD and immune reconstitution, as well as the desirable graft-versus-tumor (GvT) effect remains a great challenge for clinicians.