Immunohistology of Bone Marrow, Spleen, and Histiocytic Disorders

CD34

CD34 is a 115-kDa transmembrane sialomucin encoded on chromosome 1q32.1. Proposed functions include inhibition of differentiation, proliferation, and adhesion. Most of the studies evaluating CD34 expression have used the MY10 or the QBEND/10 monoclonal antibodies. Staining is described in blasts as membranous with some cytoplasmic staining.

Numerous benign and neoplastic proliferations of early hematolymphoid precursors express CD34. The marker is not lineage specific, and expression can be identified on lymphoid and myeloid blasts. Nonneoplastic vascular and rare stromal elements can express CD34. Neoplastic proliferations with CD34 expression include most vascular tumors, some spindle cell tumors, and hematopoietic-derived tumors described herein.

CD34 is most frequently used to quantify blasts in bone marrow sections where an aspirate was not obtainable. ^, In addition, mature megakaryocytes may express CD34 in myeloid stem cell disorders and in rare reactive conditions.

CD99

CD99 (p30/32mic2) is a cell surface glycoprotein (GP) frequently used to identify peripheral neuroepithelioma and Ewing sarcoma. The monoclonal antibody 013 recognizes a human thymus leukemia antigen and has also been found in immature terminal deoxynucleotidyl transferase (TdT)-expressing bone marrow precursors. The staining pattern is cytoplasmic.

As with CD34, CD99 may be used as a marker of immaturity in the bone marrow sections where an aspirate was not obtained. The staining is comparable with TdT; however, CD99 is less sensitive.

CD117

CD117 (C-KIT) is a 145-kDa transmembrane tyrosine kinase receptor that is the product of the KIT gene located on chromosome 4 (4q11–q12). ^, CD117 is not lineage specific, and expression is seen on numerous tissues throughout the body, including hematopoietic precursors, mast cells, and melanocytes. Staining on myeloid and erythroid precursors and the neoplastic cells of plasma cell myeloma is weak and cytoplasmic. Staining in mast cells is strong and cytoplasmic.

CD117 immunoreactivity inhibitors as anti-KIT therapies may be relevant in myeloid diseases, mast cell disease, gastrointestinal stromal tumors, and melanomas (among others).

Terminal Deoxynucleotidyl Transferase

TdT is a DNA polymerase that generates antigen receptor diversity by catalyzing the addition of deoxynucleotides to the 3′-hydroxyl terminus of the rearranged immunoglobulin heavy chain and T-cell receptor gene segments. ^, Nuclear expression is most frequent, but membranous or paranuclear dot-like positivity can be seen in some small round blue cell tumors and Merkel cell carcinoma. ^,

TdT is generally used in marrow sections to identify and quantify blasts or early normal lymphoid precursors (hematogones) when an aspirate is unobtainable.

TdT is expressed in the lymphoblasts of most acute T or B leukemia/lymphoma, a subset of acute myeloid leukemias (AMLs) and blastic plasmacytoid dendritic cell neoplasm (BPDCN). Cases of mature, high-grade B-cell lymphomas can show TdT expression, although typically focal and more variable than in TdT-expressing acute leukemias. Nonhematopoietic malignancies expressing TdT include some pediatric small round blue cell tumors, Merkel cell carcinoma, and small cell lung carcinoma.

CD33

CD33 (GP67) is a 67-kDa glycosylated transmembrane protein of the sialic acid binding immunoglobulin-like lectin family that maps to chromosome 19q13.1-3 and is thought to play a role in cell adhesion. The staining pattern is membranous.

CD33 is used to identify cells of myelomonocytic derivation and may be helpful to identify suspected myeloid sarcoma in tissue sections. Anti-CD33 targeted chemotherapy (gemtuzumab ozogamicin) is used in cases of AML and acute lymphoblastic leukemia (ALL).

CD33 is strongly expressed on myeloid precursors including granulocytes, eosinophils, monocytes, mast cells, and macrophages/histiocytes. During maturation, granulocytes and eosinophils decrease their expression of CD33 whereas monocytic or myelomonocytic cells do not. CD33 has been reported as a frequent finding in the neoplastic plasma cells by flow cytometry but only rare cases by immunohistochemistry (IHC). No expression of CD33 is reported on lymphocytes, erythroid precursors, megakaryocytes, or normal plasma cells. Both myeloid blasts and rarely neoplastic lymphoblasts may aberrantly express this marker.

Myeloperoxidase

Myeloperoxidase staining recognizes the primary granules in the cytoplasm of granulocytes, eosinophils, and their precursors. The polyclonal antibody staining pattern is cytoplasmic.

Myeloperoxidase is myeloid lineage specific but not limited to granulocytes. Cells of monocytic derivation may be weakly positive or nonreactive to the polyclonal myeloperoxidase antibody. Mast cells, plasma cells, lymphoid cells, megakaryocytes, and erythroblasts are expected to be negative for this marker.

Lysozyme

The lysozyme antibody is polyclonal and is purified from rabbit antiserum after exposure to lysozyme purified from urine from patients with monocytic leukemia. The staining pattern is cytoplasmic.

Lysozyme may be used as a granulocytic marker, although myeloperoxidase has higher specificity. The antibody stains granulocytes and granulocyte precursors, as well as monocytes and macrophages/histiocytes.

CD42b

CD42b (GP Ibα) is a GP that is part of the GP Ib-V-IX complex. ^, The staining pattern is cytoplasmic. CD42b is a robust stain in decalcified bone marrow sections with high sensitivity and specificity. Megakaryocytic markers are typically used in bone marrow sections, and rarely other tissues, to identify small immature megakaryoblasts, micromegakaryocytes, or to aid in quantification of mature megakaryocytic forms.

CD61

CD61 (β3 integrin) is a membrane-bound GP that forms a heterodimer with CD41 (GP IIb) to form the GP IIb/IIIa (CD41/CD61) complex. The staining pattern is cytoplasmic. CD61, similar to CD42b, is useful in identifying blasts with megakaryocytic differentiation or mature megakaryocytes.

CD71

CD71 (transferrin receptor-1) mediates the uptake of transferrin-iron complexes and is highly expressed on the surface of erythroid precursors including erythroblasts. The staining pattern is membranous and cytoplasmic.

CD71 is used to highlight erythroblasts and erythroid precursors with highly sensitive staining results when compared with other erythroid markers such as glycophorin A (CD235a) and hemoglobin. Mature erythrocytes, lymphocytes, and other marrow elements lack significant expression of CD71.

E-Cadherin

E-cadherin is mainly expressed by epithelial cells and was thought to be primarily an adhesion molecule. ^, Since its discovery on the developing erythroid cells, a differentiation and signal transduction function has also been proposed. The staining pattern is cytoplasmic.

E-cadherin may be used to identify nonneoplastic erythroblasts and normoblasts. Mature erythroid cells do not express this marker. E-cadherin may be downregulated during erythroleukemia on the developing erythroid cells.

Hemoglobin/Glycophorin A

Hemoglobin is a polyclonal antibody reactive with hemoglobin A and F. The staining pattern is cytoplasmic. Hemoglobin is primarily used to demonstrate the erythroid nature of the blasts of acute erythroleukemia. In addition, hemoglobin may be used to confirm the erythroid nature of the sometimes “worrisome-looking” proerythroblasts observed in patients with megaloblastic anemia, myelodysplastic syndromes (MDS), and chemotherapy-induced megaloblastoid changes. Glycophorin A is a sialoglycoprotein of red blood cell membranes. Immunohistochemical staining has similar qualities to hemoglobin A. Both hemoglobin and glycophorin A are second-line choices for erythroid identification and are less robust stains than either CD71 or E-cadherin.

CD1a

CD1a (R4, T6, CD1, FCB6, HTA1) is a transmembrane GP that forms a heterodimer with β2 microglobulin to present lipid/glycolipid antigen self-origin to T cells. ^, There are five CD1 genes located on chromosome 1 (1q23.1). CD1a is expressed on indeterminate cells (Langerhans precursors), Langerhans cells, and thymic cortical T cells. The staining pattern is membranous.

CD21

CD21 (complement C3D receptor 2) is a membrane protein on B and T cells that binds complement and acts as a receptor for the Epstein-Barr virus (EBV). The gene is located at 1q32. The staining pattern is membranous.

CD35

CD35 (CR1, C3b/C4b receptor 1 [Knops blood group]) is a membrane GP found on erythrocytes, leukocytes, glomerular podocytes, and follicular dendritic cells. The function of which is to mediate binding to particles and immune complexes activated by complement. The staining pattern is membranous.