Some malignant hematopoietic disorder is not only a disease of HSCs and hematopoietic progenitor cells, but is rather a disease of the entire HSC niche. So the interaction between HSCs and hematopoietic progenitor cells and their complex microenvironment in more details and for evaluating the role of different niche cells leading to understanding dysregulated cellular and molecular dynamics in the disorders of hematopoietic stem cell niche. These strategies aimed at preventing the development of the abnormal niche to restore normal hematopoiesis and disrupt the malignant self-reinforcing niche because self-reinforcing mechanism refers to a process where the leukemic cells create an environment that favors their survival and proliferation, ultimately leading to disease progression and relapse. This niche supports 1) their survival and proliferation while suppressing normal hematopoiesis and the leukemic cells suppressing the function of normal hematopoietic stem cells. 2) the niche provides a haven for surviving leukemic stem cells and allows them to repopulate the leukemic clone. Thus, this altered perception of micro-environmental signals along with the ability to directly remodel the niche into a self-reinforcing malignant structure might support and strengthen the malignant population with respect to normal hematopoiesis. Recent studies have highlighted the niche role in some leukemias like AML, hence it’s noticeable that in some malignant hematopoietic diseases with normal genetic state, the BM abnormality concept can be acceptable as the central and essential role. 

We know peripheral blood cells originate from hematopoietic stem cells (HSCs) that reside in fetal liver and fetal or adult bone marrow (BM) HSCs are able to self- renew which is essential for maintenance of lifelong hematopoiesis. They can differentiate to multipotent progenitors, common lymphoid and myeloid-erythroid progenitors that in turn give rise to more differentiated precursor cells. These differentiated and mature cells of erythroid, myeloid and megakaryocytic lineages are present before they are released to the circulation which in fact these disturbances can occur in single or multiple lineages.

Hematopoiesis is an orderly but complex interplay of stem and progenitor cells, growth factors, BM stromal elements, and positive and negative humoral and cellular regulators. For example, the failure of BM can  potentially occur in some critical points at the hematopoietic lineage pathways. Also you know, normal hematopoiesis depends on critical interactions that occur between stem cells and their microenvironment. In some studies have stated that the alterations in the BM microenvironment can lead to pre-leukemia and/or leukemia (figure 1). In fact, it is   the interaction between  pre-leukemic cells and its  microenvironment, since the pre-leukemia phase is usually clinical silent, but the study of these cells and their characteristics are very different which can be an important point in the diseases like pre-leukemia or cases with unexpected cytpenia. [2,3,5]

Figure 1: In bone marrow microenvironment, the hematopoiesis home has been considered to play a crucial role in both hematopoiesis and leukemogenesis. [21]

Alterations in bone marrow environment:

In principle, single lineage cytopenias may be caused by insufficient production or premature depletion of mature cells of the respective hematopoietic lineage. Additionally, it’s important to emphasize that inherited BM failure syndromes as well as acquired BM failure can present in all age groups that including some cases an isolated cytopenia precedes the development of pancytopenia. Anyhow, cytopenias affecting two or three lineages, the latter called pancytopenia. Pancytopenia: is an important clinic-hematological entity encountered in daily clinical practice which can be caused by deregulation of either cell generation or degradation. Also, it’s defined as reduction of all three formed blood components below the normal range that is simultaneous presence of anemia, leukopenia and thrombocytopenia. In this context, approximately the commonest cause of pancytopenia in various studies throughout the world has been aplastic anemia (AA). For better description, supposedly in a case of AA that is mimicking myelofibrosis, the distinction between myelofibrosis and AA lies not only in clinical symptoms but in genetic and molecule markers necessitating a nuanced diagnostic approach. A researcher stated that the primary cause of AA stems from the destruction of hematopoietic stem and progenitor cells (HSPCs)resulting from a combination of inherent genetic defects within HSPCs themselves and immune response triggered by a viral infection that is abnormally activated. So, this can be the AA cause, whether constitutional or genetic and or immune mediated destruction of stem cells (figure 2). [5-7,10]

Figure 2: This case characterized by a reduction in peripheral blood cell counts that shown predominantly hypochromic, microcytic RBCs, moderate aniso-poikilocytosis and occasional segmented neutrophil. Also few fragmented RBCs are seen and platelets aren’t seen. Meanwhile no abnormal or premature cells or parasite is seen. [10]

But In some cases BM cell counts affecting on all three lineages at the PB and BM hematopoietic disorders that perhaps the BM niche is disrupted and so how the BM microenvironment is affected by some disorders like MDS.

Figure 3: The human hematopoiesis disturbances leading to cytopenias that can affect on single or multiple lineages and be caused by cell-intrinsic or extrinsic mechanisms.

Myelodysplastic syndrome (MDS):  is recognized to arise from the acquisition of sequential mutations in HSCs that either confer clonal advantage, impair normal leukocyte function or both, namely, the low blood cell counts (cytopenias) are hallmark feature of MDS. So routine blood work may reveal anemia, neutropenia and thrombocytopenia 

(pancytopenia). As an alternative, MDS is a group of myeloid malignancies characterized by ineffective hematopoiesis with dysplastic features and a high risk for progression to acute myeloid leukemia (AML) (figures 3,4&5). [12-14]

Figure 4: MDS with isolated del (5q). A, B) BM aspirate, hypolobated megakaryocyte seen. C, D) BM biopsy, hypercellular bone marrow with myeloid proliferation associated with hypolobated or nonlobated megakaryocytes. [17]

In some patients, the expansion of a malignant clone is complemented by non-cell autonomous changes in the marrow microenvironment and immune surveillance that allow the clone’s expansion. Supposedly, the term pre-leukemia referred to a heterogeneous group of hematopoietic disorders associated with a block in myeloid differentiation and chronic cytopenias. In the pre-leukemia position appear to have an identical clonality of both myeloid and lymphoid compartments. It suggests that the clone can initiate in a very early hematopoietic stem cell. So, the mutant genes involve in ineffective dysplastic hematopoiesis pathway and/or some driver mutations produce an essential block in differentiation providing a proliferative advantage to the clone cells.

Therefore, chronic inflammation or exposure to specific environmental stimulus might allow such clones to become dominant which this is very important point.  In addition, when a single ancestor cell of the positively expanded clone obtains additional driver mutations, leukemic cells emerge. Likewise, clonal expansion occurs even in tissues that normal apparently. In this regard, the potent clonal hematopoiesis clones with: a) increasing age , b) genomic instability, c) age related inflammation and at last external environmental cues that can accumulate in the BM can reach to a known stage as clonal hematopoiesis of indeterminate potential (CHIP). Notably, when the hematopoiesis become abnormal or marked by additional mutations acquisition as well as other additional external factors, the malignancy can appear. On the other hand, mesenchymal inflammation may be the result of primary (genetic or epigenetic) alterations in mesenchymal cells or be induced by inflammatory alterations in clonal hematopoietic cells. The local inflammatory niche induces functional repression of HSCs through direct receptor-mediated genotoxic stress and/or recruitment of pro-inflammatory innate immune cells, further providing an inflammatory feed-forward loop. The inflammatory stem cell niche may drive the accelerated emergence of mutations in HSPCs through direct genotoxic signaling or the induction of replicative stress. Cells harboring poising mutations (like mutations that confer resistance to inflammation-induced apoptosis) are selected for in the inflammatory environment by virtue of their relative resistance to inflammation-associated genotoxic stress. The presence of a poising mutation, in an inflammatory environment, accelerates the acquisition of additional genetic events and transformation to AML(figure5) [4-7,14-17].

Figure 5 : MDS mesenchymal niche-facilitated clonal evolution as well as induced HSCs functional repression with acquisition of additional genetic events and severe abnormal environment that finally go to transformation into acute leukemia e.g. AML. [12]

In this way, additional alterations may confer other characteristics associated with MDS like enhanced proliferation resistance to apoptosis, morphologic dysplasia specific types, ineffective hematopoiesis and cytopenia as well. These experiments exactly depict the MDS clonal dynamics in humans isn’t clear which means the mutations appear to originate exclusively in the most primitive, stem-cell-like compartment that these abnormalities in the stem and progenitor cell compartments lead to ineffective hematopoiesis, cytopenies and dysplasia. So the disease rooted in the primitive stem and progenitor cell compartment that show a profound activation of protein synthesis machinery and increased oxidative phosphorylation. The development of cytopenies in differential expansion of specific progenitor compartments may significantly vary between different phenotypes and risk profiles of MDS. [11,16-17] 

MDS with BMF: If bone marrow fibrosis(BMF) considered in MDS, definitely is as a potential factor for determining the prognostic risk. In this case, there are two types of fibers that contribute to BMF, while an increase in reticulin fibers has a limited association with the severity of the underlying malignancy, strongly collagen fibers are linked with abnormal blood counts and poor outcomes. Reticulin fibers is often reversed after therapeutic intervention, while collagen fibrosis is less possible to alleviate with treatment. In this regard, BMF is often associated with dry tap that could hinder morphologic assessment of dysplasia. So, careful peripheral blood examination and also the slides from BM touch imprint would be helpful. Finally the prognostic impact of BMF should be factored in when deciding on transplant candidacy (as a potential treatment strategy) particularly for intermediate risk patients. [12-13,16] 

Primary myelofibrosis (PMF): In general, two phenotypes are distinguished by blood counts including a) with proliferative MF, more likely to have leukocytosis and normal to mildly low hemoglobin levels and platelets. b) cytopenic MF, more likely to have more severe thrombocytopenia, anemia and at times leukopenia. But basically the neoplasm is in the myeloproliferative process that characterized by several abnormalities including, [1] the multi-potency of the hematopoietic clonal stem cells with myeloid and lymphoid differentiation although an absolute lymphopenia has been described in peripheral blood.  [2] the clonal hematopoiesis progressive dominance over normal polyclonal hematopoiesis resulting in the overproduction of one or more of the mature blood elements [3] a striking involvement of the megakaryocytic lineage with hyperplasia ad dysplasia resulting an excessive production of cytokines and chemokines [4] the mutations presence in the JAK2 and in the absence of consistent cytogenetic abnormality [5] hematopoietic progenitors hypersensitivity to growth factors.  Moreover, it’s mentionable that myelofibrosis associating the clonal myelo-proliferation is a multifactor process resulting from alterations of fibroblasts leading to the modified expression of adhesion molecules and to an increased deposit of extra cellular components which can be the consequence of intramedullary release of growth factors by the malignant hematopoietic clone particularly by dysplastic megakaryocytes. Also, in PMF, whereas the primitive molecular event is still unknown the specificity of the pathological process would result from alterations in the cross talk between hematopoietic and stromal cells. In this process stromal cells are conditioned by growth factors produced by malignant hematopoietic cells and by acquiring new properties reciprocally, so stromal cells create a pathological microenvironment which takes part in the clone maintenance and its development, leading to an imbalance that compromises normal hematopoiesis. [8-10,20]

Figure 6: Cellular interactions involved in myelofibrosos. In left half: the figure broadly depicts cells and cell-derived factors which callobrate to stimulate fibrosis. The right half demonstrates the sequence of events that follow stimulation of MSCs in marrow niches culminating in myelofibrosis [8]

In this case, the abnormal trafficking of CD34+hematopoietic progenitors and endothelial precursors that features PMF allowing them to escape from the niche to the circulation with homing to the liver and spleen as well; namely, the malignant HSCs can survive in anatomical sites like liver and spleen that do not able to support normal hematopoiesis. (figures 6& 7)

Figure 7: PMF stem cells moving from niches to niches. These stem cells would migrate to spleen/liver where newly created or reinitialized vascular niches would favor their homing and differentiation resulting in an extra-medullary hematopoiesis.at the organs [9].

It’s noticeable that recent studies using different models of hematopoietic malignancies associated with BM fibrosis were able to mechanistically elucidate how malignant hematopoietic cells and pro-inflammatory cytokines activate stromal cells in the BM, support their fibrotic and secretory activity and influence their reduced hematopoiesis supporting capacity. Also, the malignant hematopoietic cells in murine models of MPN and AML can damage Nes-MSCs and nerve cells and in these models’ hematological malignancy development created neutrophilic changes in the BM niche, that affected the perivascular MSCs and the function of the HSC niche. Some of them suggested that dysregulation of the interaction between HSCs and hematopoietic progenitor cells and distinct stromal cell populations either in malignant hematopoietic disorders or in inflammatory processes, disrupts the tightly regulated hematopoiesis process in the BM niche and favours a secretory phenotype of stromal cells with decreased hematopoiesis supporting capacity. Therefore, we can say that: [1] By combining a clonal proliferation and a mobilization of hematopoietic stem cell(s) with BM marked alterations and spleen stroma, generally PMF illustrates a unique model in which deregulation of hematopoietic stem cell niche can play a key role in the myeloproliferative process (figures 6&7).  [2] There is an imbalance between endosteal and vascular niches in BM and spleen participates in the development and maintain of the clonal hematopoietic stem cell proliferation leading to the neoplasm. Hence, HSC deregulation is a key step in the process. Notably, fibrosis of the marrow is orchestrated by cells that are either malignant and clonal or non-neoplastic and non-clonal. [2-3,8-11]     

It is hypothesized that myeloid malignancies like PMF can be worsened by deregulation of the BM microenvironment including MSCs and their progeny. Hence, the alterations in PMF contribute in creating an abnormal microenvironment that participates in the maintenance of the neoplastic clone leading to a misbalance disfavouring normal hematopoiesis in return or simultaneously stromal cells constituting the niches are modulated by hematopoietic cells resulting in stroma dysfunctions. Also, the MDS findings confirm not only the clonal involvement of phenotypically distinct and conserved hematopoietic stem and progenitor cell compartments but also imply that stem-ness and lineage restriction remain restricted to the same HSC and progenitor compartments as in normal hematopoiesis. Meanwhile, MDS stem cells similar to normal HSCs and at distinction from MDS progenitors are highly quiescent that meaning they are in a non-dividing state but can be activated. So this quiescence is a key factor in treatment resistance, namely, cellular quiescence has been identified as a mechanism allowing escape from therapeutic strategies biased towards targeting more actively cycling cells, hence the identification of rare and distinct MDS stem and progenitor cells offers opportunities for better identifying, characterizing and more specifically and efficiently targeting the critical disease-propagating cells in MDS. In reality, these changes create a self-reinforcing cycle of damage which derives lineage biases and regenerative response defects from the blood production system that localized inflammation drives the constitutive activation of emergency myelopoiesis pathways specially from MPPs and old HSCs, reinforcing myeloid production in the erythroid and lymphoid commitment expense which may actively participate in the generation of self-reinforcing malignant niches at the expense of normal HSCs. [1-2,16-19] Therefore, monitoring the fate of these cells can provide a sensitive and early biomarker for predicting relapses and the long term effective-ness of different therapies.