Neoplastic Monocyte-Derived Fibrocytes Induce Bone Marrow Fibrosis in Patients with Primary Myelofibrosis

Progressive bone marrow (BM) fibrosis is a defining feature of primary myelofibrosis (PMF). The extent of BM fibrosis has been closely associated with the clinical manifestations of advanced disease, including anemia and thrombocytopenia, rendering it an important prognostic indicator. Mesenchymal stromal cells (MSCs) have long been regarded as the exclusive inducer of BM fibrosis. However, using xenograft and transgenic animal models, we and other investigators demonstrated that neoplastic monocyte-derived fibrocytes independently contribute to the induction of BM fibrosis in PMF. Notably, our data revealed that PMF fibrocytes but not MSCs, carry pro-fibrotic signaling pathways that are absent in healthy individuals’ fibrocytes or MSCs. Despite these findings, the precise contribution of BM fibrocytes and MSCs to the induction of BM fibrosis in patients with PMF remains elusive.

To determine which of two subsets of collagen-producing cells are the main contributors to the induction of BM fibrosis in PMF, we analyzed BM biopsies of patients with PMF and assessed whether their distribution spatially correlated with areas of fibrosis. We analyzed formalin-fixed paraffin-embedded BM trephine biopsies obtained from previously untreated patients with PMF (n = 85) and age-matched healthy donors (n = 6). Using a 7-plexed fluorescence immunohistochemistry (IHC) in conjunction with hematoxylin/eosin, Masson's trichrome, and Gomorri's silver stains we analyzed those BM biopsy specimens. To ensure comprehensive assessment, we imaged serial whole-tissue sections from each patient at 400x resolution, meticulously aligning and carefully correcting for bone trabeculae and any artifacts. To quantitate the spatial distribution of individual cells, collagen fibers, and reticulin fibers, we developed a set of analytical algorithms based on a U-Net neural network in Visiopharm software.

Our analysis revealed a 6-fold higher density of fibrocytes, defined as CD45⁺/CD68⁺/procollagen type I⁺ cells, in the BM of patients with PMF (P=0.0003) compared to normal controls. By contrast, the number of MSCs, defined as CD45^-/CD90⁺/CD105⁺ cells, in the PMF BM was not different from the number of MSCs in normal BM. After dividing the area of each BM biopsy into a 100 μm regular hexagonal grid, the observed spatial distribution of fibrocytes strongly and positively correlated with the areas covered by reticulin and collagen fibers (P<0.0001 in both). Furthermore, spatial analysis showed that increased densities of fibrocytes were associated with an increased number of fibrotic cellular zones in the BM tissue of PMF patients.

To investigate the clonality of PMF BM fibrocytes, we conducted DNA in situ hybridization (ISH) analysis, employing two sets of locus-specific probe pairs. The DNA-ISH analysis provided valuable insights, revealing that fibrocytes obtained from PMF patients with trisomy 9 or 20q deletion carried the same corresponding cytogenetic abnormalities, indicating that PMF fibrocytes indeed originate from the neoplastic PMF clone.

Because increased peripheral blood monocyte counts, commonly detected in patients with PMF, are associated with a poor prognosis, we sought to determine the potential correlation between BM fibrocyte and circulating monocyte counts. As expected, we found a significant positive correlation between BM fibrocyte counts and the percentage of circulating monocytes (P=0.001). Moreover, we also found that fibrocyte numbers correlated with the levels of the plasma cytokines pentraxin 3, known to induce the differentiation of monocytes into fibrocytes, and matrix metalloproteinase 2, as assessed by using magnetic immunoassay, suggesting that neoplastic fibrocytes induce BM fibrosis in PMF and that fibrocyte quantification could serve as a valuable indicator of disease severity and prognosis.

Taken together, our quantitative imaging analysis of BM biopsy tissue from treatment naïve PMF patients revealed that neoplastic fibrocytes, rather than MSCs, are the main contributors to the induction of BM fibrosis in PMF. Our findings underscore the importance of BM fibrocytes in the pathogenesis of PMF and warrant further investigation to identify novel therapeutic agents that will either inhibit fibrocyte differentiation or block their pro-fibrotic function.