pax-5

The pax-5 gene is essential for B-cell differentiation. There are at least four isoforms, of which pax-5a has been most studied. Pax-5  encodes the 50 kDa B-cell specific activator protein, BSAP. Pax-5 is expressed by pro-, pre- and mature B cells, but is downregulated during terminal differentiation of plasma cells. Pax-5 influences the expression of other B-cell specific genes, including CD19 and CD20 and CD79a, preceding the expression of CD20. Pax-5 is silenced at the plasma cell stage under the influence of B-lymphocyte-induced maturation protein-1 (PRDM1)⁵.

Pax-5 is expressed during mouse embryogenesis within the developing brain in a way that is temporarily and spatially tightly condoled. Pax-5 deficient mice show deformation of the mid-brain. Expression in human embryogenesis occurs in the mesoencephalon and spinal cord⁶.

Antibodies are reactive in formalin-fixed paraffin-embedded tissue. Immunoreactivity is nuclear.

Immunohistochemical expression

• squamous epithelium^2,6, skin appendages⁶, endothelial cells⁶, fibroblasts⁶ and other stromal cells⁶, respiratory epithelium⁶ and pneumocytes⁶ are all negative.

  There is positivity in the epithelium of the rete testis and epididymis.

• Non-neoplastic lymphoid tissue showed the same distributions for pax-5 and CD20 immunoreactivity. There is positivity of germinal centres, mantle zone (strong), monocytoid B-cells (weak), and intra-epithelial lymphocytes². Interfollicular blasts have variously been reported as positive² and negative³. Bone marrow showed additional pax-5 positivity due to reactivity of hematogones. In B-cell lymphomas, there appears to be an excellent combination of sensitivity and specificity. Note that some studies used monoclonal^1,2 and some polyclonal³ antibodies; the latter may be less sensitive. Results for CD20 on the same cases are given for comparison:

	pax-5	CD 20
pre-B ALL / B-cell lymphoblastic lymphoma	20/20 (all strong staining)1, 3/32, 1/2 (weak staining)3	7/20 (4 weak and 3 strong staining)1
CLL/SLL	38/38 (all strong staining)1, 13/13 (4 strong, 6 moderate and three weak staining)3, 2/24	38/38 (26 weak and 12 strong staining)1
lymphoplasmacytic lymphoma	8/8 (2/8 weak and 6/8 strong staining. Cells of plasma cell morphology showed weaker staining)1, 5/52, 1/2 (moderate staining)3	8/8 (3 weak and 5 strong staining)1
splenic marginal zone lymphoma	4/4 (all strong staining)1	4/4 (all strong staining)1
hairy cell leukaemia	12/12 (all strong staining)1	12/12 (all strong staining)1
plasma cell myeloma	2/39 (both weak staining. two other cases showed borderline staining. Three of four cases also showed weak positivity for CD20)1, 0/23	3/39 (all weak staining)1
CD20-positive myeloma (10-15% of myelomas are positive for CD20)	18/255
MGUS	0/21	0/21
solitary plasmacytoma of bone	0/11	0/11
extraosseus plasmacytoma	0/11, 0/42, 0/33	0/11
heavy-chain disease	1/2 (weak staining)1	0/21
mucosa associated lymphoid tissue lymphoma	14/14 (all strong staining)1, 5/52, 7/12 (1 strong, 3 moderate and 3 weak staining)3	14/14 (all strong staining)1
nodal marginal zone lymphoma	2/2 (both strong staining)1, 2/24	2/2 (both strong staining)1
follicular lymphoma	64/64 (all strong staining)1, 3/32, 20/21 7 strong, 11 moderate and 2 weak staining)3, 2/24	64/64 (all strong staining)1
mantle cell lymphoma	26/26 (all strong staining)1, 3/32, 21/21 (13 strong, 7 moderate and 1 weak staining)3, 2/24	26/26 (all strong staining)1
diffuse large B-cell lymphoma	95/99 (5 weak and 90 strong staining. Two cases of anaplastic morphology were pax-5 positive / CD20 negative. All four pax-5 negative cases were cIg+ / CD20-/CD45-; two of them were CD138+)1, 6/62, 19/24 (10 strong, 7 moderate and 2 weak staining)3, 9/104	93/99 (10 weak and 83 strong staining)1
TCRBCL	4/44
mediastinal (thymic) large B-cell lymphoma	2/2 (both strong staining)1	2/2 (both strong staining)1
intravascular large B-cell lymphoma	2/2 (both strong staining)1	2/2 (one weak and one strong staining)1
pleural effusion lymphoma	1/1 (strong staining)1	1/1 (weak staining)1
Burkitt lymphoma	5/5 (all strong staining)1, 1/12, 1/2 (moderate staining)3	5/5 (all strong staining)1
Classical Hodgkin lymphoma	112/117 (106 weak and 6 strong staining)1, 28/31 (the intensity of staining was extremely variable)2, 5/14 (focal nucleolar staining)3, 27/294	25/117 (all weak staining)1, 8/282
lymphocyte predominance Hodgkin lymphoma	58/58 (49 weak and 9 strong staining)1, 10/10 (staining more extensive and strong than in classical Hodgkin lymphoma)2, 2/4 (both weak staining)3, 15/154	58/58 (24 weak and 34 strong staining)1
peripheral T-cell lymphoma	0/151, 0/202, 0/123, 0/174	0/151
anaplastic large cell lymphoma	0/111, 0/19 (6 null and 13 T-cell type)2, 0/83, 0/74	0/111
T-cell ALL / lymphoblastic	0/201, 0/22, 0/33	0/201

Pax-5 is commonly expressed by Merkel cell carcinoma and small cell carcinoma, two small blue round cell tumours that appear in the differential diagnosis of lymphoma, but not in other small blue round cell tumours⁶:

	Merkel cell carcinoma	29/31 (4 diffuse strong, 13 diffuse moderate, 11 diffuse weak, 1 focal moderate. 3 cases subjected to flow cytometry were negative for CD19)6
	Small cell carcinoma	22/30 (17 of 23 in lung, 2 of 3 in bladder, 2 of 3 metastatic to lymph nodes and 1 of 1 case in colon: 2 diffuse strong, 12 diffuse moderate, 6 diffuse weak, 2 focal moderate)6
	Carcinoid tumour	0/17 (7 in lung, 10 in gastrointestinal tract)6
	PNET/Ewing's sarcoma	0/36
	Neuroblastoma	0/26

• Pax-5 and pax-6 are expressed by about 70% of medulloblastomas as detected by Northern blot analysis⁷.

Diagnostic utility

Identification of B-cell lymphomas including:

• the detection of pre-B cells, where it serves as a surrogate of CD19. All cells committed to B-cell lineage express Pax-5, including those mixed lineage acute lymphoblastic leukemia that are positive for CD19. However, note that other small round blue cell tumours such as Merkel cell carcinoma and small cell carcinoma may be immunoreactive.

• the differentiation of Classical Hodgkin lymphoma (positive for pax-5) from T and "null" cell anaplastic large cell lymphoma (negative for pax-5).

• the differentiation of lymphoplasmacytic lymphoma/plasmacytoid differentiation in B-CLL, lymphoplasmacytic lymphoma/Waldenstrom macroglobulinaemia and mucosa associated lymphoid tissue lymphoma (positive for pax-5) from plasma cell myeloma (negative for pax-5). However, care needs to be take with the minority of myelomas that are CD20+/Pax-5+⁵.

References

¹ Torlakovic, E., G. Torlakovic, et al. “The value of anti-pax-5 immunostaining in routinely fixed and paraffin- embedded sections: a novel pan pre-B and B-cell marker.” Am J Surg Pathol 2002; 26(10): 1343-50. with clarificatrions kindly provided by Dr Torlakovic.

² Foss, H. D., R. Reusch, et al. (1999). “Frequent expression of the B-cell-specific activator protein in Reed- Sternberg cells of classical Hodgkin's disease provides further evidence for its B-cell origin.” Blood 94(9): 3108-13.

³ Krenacs, L., A. W. Himmelmann, et al. (1998). “Transcription factor B-cell-specific activator protein (BSAP) is differentially expressed in B cells and in subsets of B-cell lymphomas.” Blood 92(4): 1308-16.

⁴ Torlakovic, E., A. Tierens, et al. (2001). “The transcription factor PU.1, necessary for B-cell development is expressed in lymphocyte predominance, but not classical Hodgkin's disease.” Am J Pathol 159(5): 1807-14.

⁵ Lin, P., M. Mahdavy, et al. (2004). "Expression of PAX5 in CD20-positive multiple myeloma assessed by immunohistochemistry and oligonucleotide microarray." Mod Pathol 17(10): 1217-22.

⁶ Dong HY, Liu W, Cohen P, et al. B-cell specific activation protein encoded by the PAX-5 gene is commonly expressed in merkel cell carcinoma and small cell carcinomas. Am J Surg Pathol 2005; 29:687-92

⁷ Kozmik Z, Sure U, Ruedi D, et al. Deregulated expression of PAX5 in medulloblastoma. Proc Natl Acad Sci U S A 1995; 92:5709-13

This page last revised 18.8.2005.

©SMUHT/PW Bishop