CD 74 (LN2)

Ancell/Pharmingen (MB741), American Research Products (MB3), Biodesign (BU43, BU45), Biogenex (LN-2), Cymbus Bioscience (BU45), Harlan Seralab/Accurate (2G5), ICN Biomedicals (LN-2), Dako (LN2), Immunotech (LN2), Novocastra (LN-2), Pharmingen (LN2), RDI (BU45, LN2), Serotec (BU 45), Sigma (LN2) and Zymed (LN2).

This antibody is applicable to formalin-fixed, paraffin-embedded tissue sections, frozen sections and cytological preparations. Immunoreaction may be improved with microwave antigen retrieval in citrate buffer.

The CD 74 antigen represents a membrane-bound subunit of the MHC class II-associated invariant chain (Wilson et al, 1993) that is encoded by the gene located on chromosome 5 region q31-q33 (Moller, 1995). The monoclonal antibody LN2 recognizes nuclear and cytoplasmic antigens of molecular weights 35 kD and 31 kD respectively in routinely processed tissues. MB-3, another mononuclear antibody, is thought to be identical to LN2. LN2 reacts with about 50% and 75% of activated and resting L20+ B cells in the peripheral blood and tonsils respectively. LN2 is positive in less than 3% of CD 3+ T cells. Very weak staining may be seen on circulating monocytes and granulocytes are negative.
In lymph nodes, LN2 positivity is seen primarily in germinal center and mantle cells. Staining is strongest in small germinal center cells and in mantle cells. Plasma cells are not labelled. The vast majority of cells in the interfollicular areas are negative except for interdigitating dendritic reticular cells, which are often strongly positive. Besides distinct staining of the nuclear membrane, there may be diffuse or paranuclear cytoplasmic staining, the pattern of staining being similar in both fixed and frozen tissue sections.
Thymocytes are negative for LN2 but thymic dendritic cells may often be positive. Other cells that may be positive for LN2 include sinusoidal histiocytes, epithelioid histiocytes, splenic red pulp histiocytes and Langhanstype giant cells. In addition, some epithelial cells and corresponding carcinomas may be positive but the staining of LN2 in these cells is often diffuse in the cytoplasm and the distinctive nuclear membrane staining is not observed (Epstein et al, 1984; Okon et al, 1985).

The LN2 antibody stains about 90% and 20% of low-grade B- and T-cell lymphomas respectively. In high-grade lymphomas, the corresponding figures are 85% and 75% respectively so that its value as a discriminator is less in large cell lymphomas. The pattern of labeling is also different. In small lymphocytic lymphoma, LN2 shows either nuclear membrane or dot-like cytoplasmic positivity whereas, in small cleaved cells nuclear membrane staining is the predominant pattern. In the mixed cell lymphomas and large cell lymphomas, LN2 stains the nuclear membranes of the small cleaved cells but only some of the larger cells, exhibit cytoplasmic staining, the minority displaying bright cytoplasmic globules.
Reed-Sternberg cells also stain with LN2, exhibiting cytoplasmic, cytoplasmic membrane and nuclear membrane staining in about two-thirds of cases. The antigen is expressed in about 60% of precursor B-cell ALLs/LBLs, about 50% of AMLs (excluding FAB M6 AML), most cases of CML, granulocytic sarcomas and true
histiocytic sarcomas (Norton & Isaacson, 1989a, b).
A recent study showed that LN2 antigen is strongly expressed by cells of malignant fibrous histiocytoma (MFH) but not atypical fibroxanthoma (AFX) (Lazova et al, 1997). LN2 immunoreactivity appears to distinguish between these two histologically similar yet biologically distinct tumors with a high degree of statistical significance. LN2 has also been observed to label some epithelial tumors, including adenocarcinoma of the uterus, squamous cell carcinoma of the lung and transitional cell carcinoma of the bladder (Epstein et al, 1984).

LN2 is immunoreactive in formalin-fixed, paraffin-embedded tissue sections. It is only poorly reactive in ethanol-fixed tissues and B5 fixation produces reduced positivity and a high background staining. The staining in B5-fixed tissues tends to be of the nuclear membranes and the perinuclear cytoplasm of B cells whereas, in formalin-fixed tissues, paranuclear dot-like globules are more common. Trypsinization destroys LN2 reactivity and neuraminidase treatment does not affect it (Yoshino et al, 1990). Given its contrasting immunoreactivity, it is possible that other applications of LN-2 are yet to be discovered. Benign or neoplastic B-cell tissue is recommended for optimization of this antibody.

•Epstein AL, Marder RJ, Winter JN, Fox RI 1984. Two new monoclonal antibodies (LN1 and LN2) reactive in B5 formalin fixed, paraffin-embedded tissues with follicular center and mantle zone human B lymphocytes and derived tumors. Journal of Immunology 133:1028-1036.

•Lazova R, Moynes R, May D, Scott G 1997. LN-2 (CD74). A marker to distinguish atypical fibroxanthoma from malignant fibrous histiocytoma. Cancer 79: 2115-2124.

•Moller P 1995. CD74 workshop panel report. In: Schlossman SF (ed) Leucocyte typing V. White cell differentiation antigens. Oxford: Oxford University Press, p 568.

•Norton AJ, Isaacson PG 1989a. Lymphoma phenotyping in formalin-fixed and paraffin wax-embedded tissues. I. Range of antibodies and staining patterns. Histopathology 14:437-446.

•Norton AJ, Isaacson PG 1989b. Lymphoma phenotyping in formalin-fixed and paraffin wax-embedded tissues. II. Profiles of reactivity in the various tumour types. Histopathology 557-579.

•Okon E, Felder B, Epstein A et al 1985. Monoclonal antibodies reactive with B lymphocytes and histiocytes in paraffin sections. Cancer 56:95-104.

•Wilson KM, Labeta MO, Pawelec G, Fernandez N 1993. Cell-surface expression of human histocompatibility leucocyte antigen (HLA) class II-associated invariant chain (CD74) does not always correlate with cell-surface expression of HLA class II molecules. Immunology 79: 331-335.

•Yoshino T, Hoshida Y, Murakami I et al 1990. Comparison of monoclonal antibodies reactive with lymphocyte subsets in routinely fixed paraffin-embedded material: flow cytometric analyses, immunoperoxidase staining and influence of fixatives. Acta Medica Okayama 44:243-250.

Manual of diagnostic antibodies for immunohistology / Anthony S.-Y. Leong, Kumarasen Cooper, F. Joel W.-M. Leong.