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CD11c is an integrin glycoprotein expressed by mononocytes and macrophages, neutrophils, myeloid dendritic cells and a small subset of lymphocytes. Monoclonal antibodies to CD11c have proven especially useful in the diagnosis of Hairy Cell Leukaemia, a B cell malignancy. Other haematological malignancies that express CD11c antigen include acute myeloid with a monocytoid component, systemic mastocytosis and lymphomas derived from histiocytes and dendritic cells. Although antibodies to CD11c have been available since the mid-1980s, the new NovocastraTM antibody 5D11 is the first to recognise a formalin-resistant epitope.


CD11c, a member of the Leucocyte Function Associated (LFA) family of molecules, is a 150 kD type I transmembrane glycoprotein which is also known as integrin chain alpha X (aX), leucocyte surface antigen p150,95 and CR41. CD11c, CD11a and CD11b are heterodimeric glycoproteins with distinct alpha chains which share a common beta subunit, CD18 (p95).1 These LFA molecules (Table 1) all have roles in cell-cell interactions.

The 25kb gene encoding CD11c is located on chromosome 16p11. The CD11c protein has 1,144 amino acids (1,081 extracellular; 26 transmembrane and 29 cytoplasmic) with seven repeating integrin domains.2

CD Designation

Other Names


MW (kD)


LFA-1 alpha subunit; integrin L subunit ITGAL

Integrin alphaL



Mac-1; integrin alphaM subunit; ITGAM; CR3

Integrin alphaM



P150,95 alpha subunit; integrin alpha X subunit; ITGAX; CR4

Integrin alphaX



Integrin beta2 subunit

Integrin beta2


 Table 1. Details of the LFA family of molecules

Normal Cellular Expression of CD11c

The CD11c antigen is mainly expressed on myeloid cells. In peripheral blood it is expressed at highest density on monocytes, natural killer cells and myeloid dendritic cells, at moderate levels on granulocytes and low levels on subsets of lymphocytes.1,3 CD11c is expressed by a subset of activated, virus-specific, cytotoxic T cells that have potent antiviral effects. In normal bone marrow, CD11c is primarily expressed by myeloid cells with strongest expression on monocytes and tissue macrophages (Figure 1). It is also seen on monocyte progenitors in the marrow (monoblasts and promonocytes). CD11c antigen is also expressed at high levels on tissue macrophages, including sinus histiocytes of lymph node, tonsil and spleen, macrophages in the cortex and medulla of thymus, Kupffer cells in the liver, elongated inter-tubular cells in the kidney, Langerhans’ cells in the skin and alveolar macrophages in the lung.4

Function of the CD11c Antigen

CD11c / CD18 has an important role in cytotoxic T cell killing, in the adhesion of monocytes and neutrophils to the endothelium and induces cellular activation. CD11c binds multiple ligands including complement factor fragment iC3b (a key component in the innate immune defense), fibrinogen, CD54 (ICAM-1) and bacterial lipopolysaccharide.5-8 CD11c and CD11b bind similar ligands, seem to have similar functions and are co-operative.

CD11c Antibodies

The CD11c cluster was defined at the 3rd Human Leucocyte Differentiation Antigen Workshop in Oxford in 1986.1 The CD11c cluster includes antibodies 3.9, B-ly5, B-ly6, BU15, HC1/1, KB23, KB90, KiM1, L29, LeuM5 and S-HCL3. 5D11, a new Novocastra CD11c antibody, is the only CD11c antibody that recognises a formalin-resistant epitope, making this antibody applicable to routinely processed formalin fixed tissue biopsies.

CD11c Expression in Inherited and Neoplastic Disorders

Leucocyte Adhesion Deficiency

Leucocyte Adhesion Deficiency syndrome (also known as LAD or LFA-1 deficiency) is an inherited autosomal recessive disorder resulting from congenital absence of one of the members of the LFA family of molecules. This is most commonly the CD18 beta-chain that combines with the CD11a, CD11b and CD11c alpha-subunits. Affected individuals suffer from recurrent bacterial infections and may be present in the neonatal period with infections of the umbilical stump. The susceptibility to infections is a result of defective neutrophil and monocyte function. Children with LAD rarely survive to adulthood. The diagnosis of LAD can be made with antibodies to the LFA family of molecules, including CD11c.

Haematological Maligancies

a. Hairy Cell Leukaemia

Hairy Cell Leukaemia (HCL) is an indolent, chronic B-cell lymphoproliferative disorder comprising approximately 3% of all adult leukaemias.7 Hairy cells are clonal expansions of mature, activated B-cells which express pan-B cell associated antigens such as CD19, CD20, CD22, CD79a and CD79b. HCL has a characteristic B-cell phenotype expressing CD11c, the interleukin-2 receptor CD25 and the human mucosal lymphocyte antigen CD103.8,9 This phenotype is useful for distinguising HCL from other small cell B cell lymphoproliferative disorders, a critical distinction as there are specific potentially curative therapies available, namely Cladribine and Pentostatin.

Until recently CD11c antibodies have been limited to use on fresh frozen tissue, flow cytometry or cell smears (Figure 2). The availability of clone 5D11 allows the diagnosis, and monitoring of residual disease following therapy, in formalin-fixed spleen or bone marrow (Figure 3).10

b. B-cell Lymphoproliferative Disorders

CD11c is rarely expressed by small B-cell lymphoproliferative disorders other than HCL. Rare cases of chronic lymphocytic leukaemia (CLL) / small lymphocytic lymphoma and splenic marginal zone lymphoma have been reported to be positive, but, in general, the CD11c expression is weak.11 Flow cytometric analysis has demonstrated that the intensity of CD11c expression in HCL to be 30-fold greater than in CLL / small lymphocytic lymphoma and splenic marginal zone lymphoma12 demonstrating its diagnostic utility in the differential diagnosis of small B-cell malignancies. A small number of cases of lymphoplasmacytic lymphoma/ Waldenstrom macroglobulinemia have also been shown to have some weak CD11c expression by flow cytometry.12 However, in a recent study using 5D11 in fixed tissue biopsies, with or without decalcification, the CD11c is virtually always negative in these non-hairy cell small B-cell lymphoproliferative disorders.10 Very rare cases of diffuse large B-cell lymphoma have been reported to express CD11c antigen.

 c. Acute Myeloid Leukaemia

Expression of CD11c by monocyte progenitors is useful for the sub-classification of acute myeloid leukaemia (AML). Up to 50% of AML with monocytoid differentiation (acute monoblastic leukaemia, acute monocytic leukaemia and acute myelo-monocytic leukaemia) are CD11c positive, as demonstrated by immunophenotyping of cell smears and by flow cytometry.14, 15 Other types of AML, and specifically acute promyelocytic leukaemia, which express monocyte associated antigen CD68, are CD11c negative. Assessment of bone marrow trephine biopsies of AML with 5D11 has shown positivity in approx-imately 25% of cases. All of these had morphological evidence of monocytoid differentiation.

 d. Systemic Mastocytosis

Mastocytosis is a neoplasm with proliferation and accumulation of abnormal mast cells in one or more organ system of the body including the skin, bone marrow, liver and spleen. When there is significant bone marrow involvement the condition is termed systemic mastocytosis. Normal mast cells characteristically express CD9, CD68 and CD117 antigens, and contain high levels of tryptase, whereas pathological mast cells in mastocytosis show aberrant expression of CD2 and CD25 antigens and over-express cell surface complement-associated molecules. Specifically CD11c and CD35, complement receptors, and CD59, the complement regulatory molecule, are increased in mastocytosis.16, 17 This aberrant expression of CD11c, together with CD2 and CD25, can be utilized in the diagnosis and moni-toring of mastocytosis in tissue biopsies. Clone 5D11 can be used for the assessment of bone marrow trephines in patients with suspected systemic mastocytosis and for disease monitoring (Figure 4).

 e. Histiocytic and Dendritic Cell Neoplasms

Tumors of histiocytes and dendritic cells are rare and consequently there are few studies of the neoplastic cell phenotype. The non-neoplastic counterparts of these disorders are known to express CD11c antigen. Early work on fresh tissue showed histiocytic sarcomas to be positive for CD11c as well as macrophage-associated lysozyme and CD68.15,18 A case of Langerhans’ cell histiocytosis was CD11c positive with antibody 5D11 (Figure 5).

 f. Aggressive Natural Killer Cell Leukaemia / Lymphoma

Aggressive natural killer cell lymphoma / leukaemia is a rare and highly aggressive disorder characterized by systemic proliferation of natural killer cells. Bone marrow, lymph node and splenic involvement are common. The leukemic cells have a natural killer cell-associated phenotype expressing CD2, CD16 and CD56 antigens. They are generally CD11c and CD11b positive.


CD11c antibodies are useful in the diagnosis of a number of haematological malignancies. Antibody 5D11 will prove to be a useful addition in the repertoire of diagnostic haemato-pathologists. Its particular application will be in the diagnosis and monitoring of haematological malignancies, especially HCL, in fixed tissue.


Figure 1. CD11c (clone 5D11) positive macrophages in a bone marrow trephine of a case of multiple myeloma (x1,000).
Example of CD11c (clone 5D11) positive macrophages in a bone marrow trephine of a case of multiple myeloma (x1,000).
Figure 2. Peripheral blood of HCL stained with CD11c. Shows the blood smear stained with antibody KB90 and immuno-alkaline phosphatase with Fast red substrate (x1,000).
Example of peripheral blood of HCL stained with CD11c. Shows the blood smear stained with antibody KB90 and immuno-alkaline phosphatase with Fast red substrate (x1,000).
Figure 3. Bone marrow trephine of HCL stained with CD11c (clone 5D11). Figure A. shows extensive marrow infiltration at diagnosis (x400) whilst Figure B. shows CD11c-positive residual disease following therapy (x1,000)
Two examples of bone marrow trephine of HCL stained with CD11c (clone 5D11). Figure A. shows extensive marrow infiltration at diagnosis (x400) whilst figure B. shows CD11c-positive residual disease following therapy (x1,000).
Figure 4. Bone marrow trephine of systemic mastocytosis stained with CD11c (clone 5D11) (x1,000).
Example of bone marrow trephine of systemic mastocytosis stained with CD11c (clone 5D11) (x1,000).
Figure 5. Bone marrow trephine showing CD11c positivity (antibody 5D11) of the neoplastic cells in a case of Langerhans’ Cell Histiocytosis (x 1,000).
Example of bone marrow trephine showing CD11c positivity (antibody 5D11) of the neoplastic cells in a case of Langerhans’ Cell Histiocytosis (x 1,000).


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