Morphological evaluation of monocytes and monocyte precursors in bone marrow trephine biopsies - need for establishing diagnostic criteria

Goasguen et al.1 in their recent article delineate morphological features that can be used to distinguish four stages of monocytes and their precursors on peripheral blood and bone marrow aspirate films. Evaluation of monocytic lineage cells in hematologic specimens is critical not only on peripheral blood and bone marrow aspirate films, but also in bone marrow trephine biopsies (BMTB). Evaluation of BMTB for a possible monocytic component is particularly sought in the following situations: (i) appreciation of monoblastic features in acute myeloid leukemia (AML) or acute monoblastic and monocytic leukemia (AMoL); (ii) identification and differentiation of chronic myelomonocytic leukemia (CMML) from other myeloproliferative disorders and myelodysplastic syndrome; (iii) subtyping of CMML into CMML-1 and CMML-2.

It is true that the currently evaluated immunophenotypic features are not completely specific for monocytes and monocyte precursors. However, many centers investigate BMTB samples with immunohistochemistry for CD68R and CD163.2,3 In BMTB of CMML samples, CD68R identifies two populations of monocytic lineage cells. However, CD163 appears to be a more reliable marker for early monocytic differentiation and for lineage identification in “blastic” cells of AMoL.

On BMTB, three stages of monocytic population can be identified (Figure 1):

The above mentioned monocytes and monocyte precursors appear distinct from phagocytic macrophages and dendritic cells on the CD68R immunostain. As compared to the monocytic cells, the phagocytic macrophages and dendritic cells have more abundant cytoplasm and the staining pattern is one of more uniform and intense staining of the entire cytoplasm. The dendritic processes of the dendritic cells are also highlighted by CD68R. In some of the phagocytic macrophages, the less-stained phagolysosomes are also recognizable on CD68R. It should be emphasized, however, that CD68R expression is also seen in other lineage cells/diseases, like in cells of hairy cell leukemia. Hence, a wider panel of immunostaining and documentation of negative staining with other antibodies, and correlation with other morphological features is essential.

The combined approach of morphology and immunostaining is helpful in identifying the lineage of blasts in cases of AML. In AMoL, not only are >80% of the marrow cells positive for CD68R/CD163, often the proportion of CD163 positive cells exceed the proportion of CD68R positive cells. Furthermore, in suspected cases of CMML, the approach helps in documenting the proportion of monocytic cells in the BMTB sample and in subtyping the cells in two categories - the more mature and the precursor subpopulations. The latter show finer granulation with CD68R and have less mature nuclei. This approach helps subtyping CMML into CMML-1 and CMML-2.

Figure 1.The left panel depicts a case of CMML. A greater proportion of cells are positive for CD68R as compared to CD163. The CD68R positive monocytic cells show granular positivity and the size and distribution of the granules are variable. The more mature monocytic cells have larger granules, while less mature forms have smaller/finer granules. Histiocytic/dendritic cells show intense positivity and some show dendritic processes. The right panel depicts a case of AMoL, where the majority of the cells are atypical and blastic. A greater proportion of the cells are positive for CD163 as compared to CD68R.

Though WHO criteria4,5 can be applied on BMTB samples with the above-mentioned approach, the criteria need revalidation and subtle modifications for usage on BMTB. Standardization of BMTB processing and quality assurance in BMTB-immunohistochemistry would be of paramount importance.6 There is a need for establishing morphological and immunomorphological criteria for diagnosis of hematologic neoplasms on BMTBs. On many occasions, the bone marrow aspirate quality (for various reasons such as marrow fibrosis or other technical reasons) may be suboptimal making a specific diagnosis difficult.