High abundance of circulating megakaryocytic cells in chronic myeloid leukemia in Indian patients. Revisiting George Minot to re-interpret megakaryocytic maturation: Beyond the abstract

Knowledge about how megakaryocytes (Mk) mature and platelets form has come chiefly from, (a) direct observation on bone marrow (BM), the primary site  of Mk development, and (b) studies on Mk cells grown in culture. Peripheral blood (PB) is less useful in this regard as Mk cells only uncommonly circulate and the occasional ones that normally do, are  the well-described lobulated bare nuclei, that are little more than curiosities. 

The peripheral blood of chronic myeloid leukemia (CML) cells is characterized by leukocytosis due to the outpouring of immature and mature myeloid cells with peaks of myelocytes and neutrophils. Megakaryocytic series, including micromegakaryocytes, are rarely mentioned as standard PB findings in textbooks and other accounts from the West,  reflecting a rare occurrence in CML as seen in the West. The occasional texts that do  mention circulating micromegakaryocytes,   state  that   they  may be seen in the chronic phase, be associated with, or increase, in the course of acceleration,  and when present in abundance in blast crisis, provide evidence of megakaryocytic lineage of the blasts.  

Overall, accounts from the West are not representative of what is quite apparent from even a casual examination of the peripheral blood of our CML patients -   that circulating Mk cells are far from rare or inconsequential findings and hence represent an unstudied population.  In the present study we evaluated the morphology of peripheral blood Mk cells in 324 (235 chronic, 65 accelerated and 24 blastic phases) untreated CML patients, to systematically document megakaryocytic maturation in the peripheral blood, a milieu not native to megakaryocytes.  

Observations and discussion:  Over 40% of our chronic phase patients and 2/3 of those who were in accelerated phase had, respectively, up to 4.5% and 8.5% circulating micromegakaryocytes in the peripheral blood smear.   Circulating Mk cells, up to approx.15%, were   present also in   1/3 of our blast crisis patients, higher percentages merging imperceptibly with pure megakaryocytic blast crisis.  

The main findings, reflecting   two possibly  inter-related themes of megakaroypoiesis in the peripheral blood,  were  as follows: 

  1. Attempt at platelet production  by megakaryoblasts – precocious and unsuccessful. First, megakaryocytic blasts, especially   in some cases of blast crisis, precociously   make a foray into platelet   formation and end up producing not normal platelets, but huge agranular or poorly granular cytoplasmic lobulated masses,   that break off and come to lie in the circulation. Though obvious from morphology alone, these   are  demonstrable  also by CD41 immunocytochemistry. This attempt at platelet formation continues unabated   even as the blasts   mature further, as evidenced by increasing condensation of  nuclear chromatin and   appearance of granules in cytoplasm (Figure, top two panels).  This evidence of unsuccessful effort may exist, in a considerably attenuated form in chronic phase as well. 
  2. Production of mature platelets by unilobate mature micromegakaryocytes. The platelet-producing micromegakaryocyte is the central character in the second theme of peripheral blood  Mk maturation. Seen in all phases of CML it is often the only type of circulating  Mk cell in the chronic, and sometimes the accelerated phase. Circulating micromegakaryocytes  have been accorded emphasis mostly as  part of myelodysplastic syndrome and acute myeloid leukemia in the Western literature. We, however, find   them   more commonly in CML. 
Mature, normal looking platelets can be seen getting pinched off from cytoplasmic processes of micromegakaryocytes in a manner that resembles the normal mode of platelet formation. One  or more micromegakaryocytes are seen  enmeshed in the midst of platelet masses in all CML cases with  prominent  thrombocytosis, an association we have not observed in settings of thrombocytosis in patients who do not have myeloproliferative conditions (Figure, lower panel). These observations point to an alternative path of Mk maturation and platelet formation in the peripheral blood in CML, distinct from what happens in the marrow. Express mention   that the small amount of cytoplasm that is seen in the smallest of circulating micromegakaryocytes is in the process of differentiation, has been made only, as far as we could ascertain, in George Minot’s 1922 essay on the  subject. Minot also mentions the association of platelet masses with entrapped micromegakaryocytes. 


We find it surprising that neither these observations, nor the frequency of circulating Mk cells in as many as  2/3 of CML cases recorded by Minot,  has  been mentioned in  reports or accounts of  Mk cells in CML that have ensued since, including those that have cited Minot’s paper. Quite clearly, circulating Mk cells have been seen  more as findings  rather than as clues  that when pieced together provide a perspective of Mk maturation  distinct from that based on observations on the BM. The suggestion that they are dyspoietic, evidently based on their appearance that is different from the normal, is premature; it has never been investigated and proven.  

Conclusion: We conclude that circulating Mk cells abound in our CML patients and afford a setting in which megakaryocytic maturation   in the peripheral blood can be studied. Two themes of maturation, possibly interrelated, and quite distinct from what happens in the marrow, play out to variable extents in individual patients. One centered around blast cells   prematurely attempting platelet formation, delivers to the circulation, in a wasted effort,  large cytoplasmic fragments rather than normal platelets. The other  has the well described micromegakaryocyte at its  center, and yields morphologically normal platelets. The former is floridly  dysplastic. The  latter, differing from normal in not being polyploid, is presumed dysplastic; the possibility, however, that it may just be an alternative path of effective platelet formation that  megakaryocytes follow when in circulation, has not been ruled out. Both these are exciting possibilities that need to be investigated.  Quite fortuitously our observations are but a revisit of the perceptive   exposition of George Minot on the subject, presented nearly a century ago. 

Written by: Anita Chopra and Rajive Kumar

Reference:

  1. Minot GR.  Megacaryocytes in the peripheral circulation. J Exp Med 1922;361: 1-7.

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