Difference in the distribution of tumor-infiltrating CD8+ T cells and FOXP3+ T cells between micronodular thymoma with lymphoid stroma and micronodular thymic carcinoma with lymphoid stroma

Micronodular thymoma with lymphoid stroma (MNT) is a rare thymic epithelial neoplasm subtype characterized by a micronodular tumor cell growth pattern and abundant lymphoid stroma. Micronodular thymic carcinoma with lymphoid stroma (MNCA) is considered as a malignant counterpart of MNT and exhibits a growth pattern similar to that of MNT but has histologic features reminiscent of thymic squamous cell carcinoma, such as cytologic atypia and CD5 and CD117 immunoexpression. Although both MNT and MNCA are characterized by abundant lymphoid stroma, it remains unknown whether there are differences in infiltrating lymphocytes between MNT and MNCA. We analyzed the immune microenvironment profile in eight MNT and three MNCA cases. The cell density of CD8-positive T cells was significantly higher in MNT than in MNCA, whereas that of FOXP3-positive T cells was significantly higher in MNCA than in MNT. There was no significant difference in the cell density of programmed death protein 1-positive T cells and programmed death ligand 1 expression between the MNT and MNCA cases. Our findings indicated that the immune microenvironment of MNCA differed from that of MNT and, compared with the T-cell profile of MNT, that of MNCA was more suppressive to patients′ antitumor immune response.     

Fine-needle aspiration cytology of thymic basaloid carcinoma: case studies and review of the literature

We report the features in fine-needle aspiration biopsy (FNAB) of thymic basaloid carcinomas. This is a rare neoplasm, of which there are only three documented in our hospital files. To the best of our knowledge, this is the first fine-needle aspiration (FNA) report on basaloid carcinoma of the thymus. This is a tumor in which the FNA diagnosis is difficult and the differential diagnosis is broad. We describe the cytologic features encountered in the three cases, and immunohistochemical and ultrastructural findings so as to raise awareness of this entity in the differential diagnosis of thymic neoplasms on FNABs. The cases studied included three male patients, aged 73, 65, and 50, who presented with anterior mediastinal masses, with no primary tumor elsewhere. FNAB was performed on two cases, followed by thymectomy. One case, additionally, had metastasis to a cervical lymph node, and the other two were associated with thymic cysts. The diagnoses on all three cases were thymic basaloid carcinoma.     

EBER-1 expression in thymic carcinoma

The Epstein-Barr virus-encoded small nuclear RNA, EBER-1, has been shown to be a suitable target for the in situ hybridization detection of EBV in routinely processed tissue specimens. We evaluated the presence of EBV in thymic carcinoma and invasive thymoma using EBER-1 in situ hybridization on formalin-fixed paraffin-embedded tissue sections. EBER-1 expression was demonstrated in a case of lymphoepithelioma-like thymic carcinoma, but was not detectable in other thymic carcinomas including six squamous cell carcinomas, a clear cell carcinoma and seven invasive thymomas. As reported in three previous cases of EBV-associated thymic carcinoma, lymphoepithelioma-like thymic carcinoma was shown to be closely associated with EBV in our series.     

Increased numbers of thymic and peripheral CD4+ CD25+Foxp3+ cells in the absence of CD5 signaling

It has been suggested that high affinity/avidity interactions are required for the thymic selection of Treg. Here, we investigated the role of CD5, a negative regulator of TCR signaling, in the selection and function of “naturally occurring” CD4⁺CD25⁺ Treg (nTreg). Analysis of CD5^?/? mice showed a significant increase in the percentage and absolute numbers of CD4⁺ CD25⁺Foxp3⁺ thymocytes and peripheral T lymphocytes, compared with BALB/c mice. Thymi from CD5^?/? mice showed reduced cellularity due to increased apoptosis, which preferentially affected naïve T cells. To characterize nTreg selection at the molecular level we investigated the phosphorylation of Erk, c‐Cbl, PI3K and Akt. CD5^?/? nTreg showed increased basal levels of p‐Erk compared with wild‐type nTreg. Interestingly, in response to CD3 plus CD28 costimulation, CD5^?/? naïve T cells but not CD5^?/? nTreg showed lower levels of p‐Akt. Finally, CD5^?/? nTreg were thymus‐derived and fully functional. We conclude that the enrichment of nTreg observed in the absence of CD5 signaling is due to de novo generation of nTreg and selective reduction of CD4⁺CD25^? naïve thymocytes. Furthermore, we provide new evidence supporting a potential role of CD5 in thymocyte survival, through a mechanism that may involve the phosphorylation of Akt.     

急性冠脉综合征患者胸腺新近输出CD4~+CD25~+调节性T细胞的检测及意义

目的:探讨急性冠脉综合征患者胸腺新近输出的CD4+CD25+调节性T细胞(Treg)的水平及意义。方法:实验共分3组:急性冠脉综合征(ACS)组28例,稳定型心绞痛(SA)组25例和健康对照(HCs)组24例。采用CD31作为胸腺新近输出的典型标志,用磁性细胞分离器(MACS)分离各组CD4+CD25+调节性T细胞,流式细胞分析法检测各组患者外周血CD31+Treg占Treg细胞的比例,应用实时定量PCR检测CD4+CD25+调节性T细胞中T细胞受体重排删除环的数量。结果:ACS组外周血CD31+Treg/Treg比例明显低于SA组(P=0.003)和HCs组(P<0.001)。ACS组Treg细胞中TREC的数量亦明显低于SA组(P=0.001)和HCs组(P<0.001)。而SA组和HCs组间差异均无统计学意义(P=0.106和P=0.566)。患者CD31+Treg的比例与Treg细胞中TREC的数量呈正相关(r=0.493,P=0.014)。结论:急性冠状动脉综合征患者胸腺新近输出CD4+CD25+调节性T细胞减少可能是动脉粥样斑块不稳定的原因。     

The spectrum of micronodular thymic epithelial tumours with lymphoid B-cell hyperplasia

AIMS: A rare type of thymoma, micronodular thymoma with lymphoid B-cell hyperplasia, was recently reported by Suster and Moran. Thymic epithelial tumours with a similar pattern but with varied cytological features of the tumour cells are analysed. METHODS AND RESULTS: A total of 11 cases of thymic epithelial tumours characterized by micronodular proliferation of tumour cells separated by abundant lymphoid stroma with prominent germinal centres were reviewed clinicopathologically and examined immunohistochemically. The presence of Epstein-Barr virus (EBV) genome was also examined by in-situ hybridization. Based on the morphology of tumour epithelial cells, cases were subdivided into four groups: group 1 (two cases) having spindle epithelial cells; group 2 (two cases) showing an admixture of spindle and polygonal epithelial cells; group 3 (five cases) having polygonal epithelial cells, with mild to moderate cytological atypia in four cases, and group 4 (two cases) representing lymphoepithelioma-like carcinoma. The degree of cytological atypia and the number of tumour cells positive for MIB-1 and p53 gradually increased towards group 4. The abundant lymphoid stroma in all cases contained many CD20-positive B-cells and CD3 and CD45RO-positive T-cells. CD99-positive immature T-cells were present in all cases of groups 1 and 2 and in most cases of group 3, but not in both cases of group 4 tumours. IgG, IgM and IgD-positive plasma cells and lymphocytes were also present in all cases, more prominent in those of groups 3 and 4. The EBV genome was detected in only a few lymphocytes in five cases. CONCLUSIONS: The tumours in this series belong to a distinct category of thymic epithelial tumours and each of the above groups may constitute a spectrum in the continuum of cytological atypia. The aetiological relationship of EBV with these tumours could not be proved. The lymphoid B-cell hyperplasia may result from a host immune response and may suggest a favourable clinical course of this type of tumour.     

CD5-positive mucosa-associated lymphoid tissue (MALT) lymphoma: a clinicopathologic study of 14 cases

Extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) is a B-cell neoplasm that is typically CD5 negative. We describe the clinicopathologic, immunophenotypic, and cytogenetic features of 14 cases of CD5+ MALT lymphoma. There were 9 men and 5 women (median age, 68 years; range, 34-87 years). MALT lymphoma was initially diagnosed in salivary glands (n = 4), nasopharynx (n = 2), and 1 case each in conjunctiva, thyroid, stomach, colon, skin, lung, kidney, and retroperitoneum. Two patients had localized disease; 9 had disseminated disease with generalized lymphadenopathy (n = 8), multifocal lymphoma (n = 6), or bone marrow involvement (n = 5). No staging information was available for the remaining patients. None presented with B symptoms, splenomegaly, cytopenias, lymphocytosis, monoclonal gammopathy, or elevated serum lactate dehyrogenase. Serum β2-microglobulin was elevated in 6. Morphologically, the neoplasms had features typical of MALT lymphoma being composed of small- to medium-sized cells with round to slightly irregular nuclear contours and moderate amount of cytoplasm. Lymphoepithelial lesions were noted in 4 cases. CD5 was positive in all cases by immunohistochemistry (n = 12) and/or flow cytometry (n = 11). All cases assessed were negative for cyclin D1 (13/13) and CD10 (11/11). Conventional cytogenetics in 7 cases showed trisomy 3 in 3 and diploid in 4. With a median follow-up of 71 months (range, 2-131 months), overall survival at 5 years was 100%, although 5 patients required chemotherapy. Our results show that CD5 expression is rare in MALT lymphoma, and is often associated with nongastric disease and an increased tendency to present with disseminated disease. Overall survival is excellent with appropriate therapy.     

B cells in the aged: CD27, CD5, and CD40 expression

Ageing is characterized by numerous changes in lymphocyte subpopulations. In the present paper we have focused on B cells carrying the surface markers CD27, CD5 and CD40. CD27 is considered a marker of primed (memory) cells and its engagement promotes the differentiation of memory B cells into plasma cells. CD5 is expressed on B1 cells, which are considered to be responsible for T cell-independent antibody production other than autoantibodies. The CD40 molecule binds CD40L (CD154) and is necessary for T-dependent antibody responses. Here we show that the absolute number of CD5+ and CD40+ B cells is decreased in the elderly, while CD27+ B lymphocytes only marginally decrease in centenarians. However, there is a decrease of the percentage of CD5+ B cells, an increase of CD27+ B cells, while CD40 does not change significantly. These data, together with the increased number of NK cells during aging, suggest different regulation of antibody production in the elderly which might be another example of immune remodeling with aging, based on interactions between human B and NK cells.     

CD138 (Syndecan-1) in thymic tumors: correlation with various World Health Organization types and clinical outcome

We intended to explore the relationship of CD138 with thymic tumors. A series of 64 thymic tumors were studied. Positive staining was seen in 7 of 8 (87.5%) type A, 7 of 16 (43.7%) type AB, 1 of 8 (12.5%) type B1, 1 of 5 (20%) type B2, 10 of 17 (58.8%) type B3, and 3 of 10 (30%) type C (p=0.04), respectively. In terms of subcellular location of CD138 expression, 9 of 10 (90%) CD138 positive type B3 had membranous expression, while cytoplasmic expression was identified in 7 of 7 (100%) type A, and 6 of 7 (86%) type AB (p<0.0001). Positive CD138 was noted in 20 of 31 (64.5%) cases with Masaoka stage I (p= 0.01); while negative CD138 was seen in 24 of 33 (72.7%) cases with Masaoka stages (II-IV). Tumor recurred in 4 cases (7%), all of which had negative CD138 (p=0.008). Our study suggests that CD138 could be used as an ancillary study to differentiate between WHO types. Furthermore, our findings demonstrate that advanced stage-thymic tumors as well as those with high recurrence rate tend to display a reduced expression of CD138. However, more studies with larger cohort and longer follow-up are warranted to validate the clinical utility of CD138 to assess clinical behavior and prognosis of thymic tumors.     

IL-10 is predominantly produced by CD19(low)CD5(+) regulatory B cell subpopulation: characterisation of CD19 (high) and CD19(low) subpopulations of CD5(+) B cells

IL-10 production by CD19(+)CD5(+) B cells was investigated, by determining the expression levels of CD19, a classical B cell marker. Peripheral mononuclear cells were stained with fluorescence-conjugated anti-CD5, anti-CD19, anti-IL-10, and Annexin V. Interestingly, IL-10-producing B cells were found to be localised within the CD19(low)CD5(+) B cell subset. Apoptotic changes were also observed mainly in CD19(low) cells among B cells. Thus, CD5(+) B cells should be classified as CD19(high) and CD19(low) cells, and the immunological significance of CD19 for the IL-10 production by CD5(+) B cells requires further studies.     

Waldenstrom macroglobulinemia with CD5+ expression presented as cryoglobulinemic glomerulonephropathy: a case report

Waldenstrom macroglobulinemia (WM) is a B-cell lymphoproliferative disorder associated with bone marrow involvement of lymphoplasmacytic lymphoma (LPL) and an IgM monoclonal gammopathy. Generally B-lymphocytes in LPL do not express CD5 that is important for differential diagnosis of B-cell lymphoproliferative disorders. In WM, various renal diseases and type I cryoglobulinemia are well described separately, but cryoglobulinemic glomerulonephropathy is very rarely reported. A 61-yr-old woman complained of generalized edema, cyanosis of the extremities in cold weather, visual disturbance, and pancytopenia. Bone marrow and renal biopsy showed CD5+ expressing B-cells and cryoglobulinemic glomerulonephropathy. With the diagnosis of WM, she received cyclophosphamide, doxorubicin, vincristine and prednisolone chemotherapy and got complete remission. Here, we report a rare case of WM associated with unusual expression of CD5+ B-lymphocytes and cryoglobulinemic glomerulonephropathy, and emphasize the importance of the clinical features in differentiating CD5+ B-cell lymphoproliferative disorders.     

Thymic microenvironment and NZB mice: the abnormal thymic microenvironment of New Zealand mice correlates with immunopathology

There are distinct microenvironmental abnormalities of thymic architecture in several murine models of SLE defined using immunohistochemistry and a panel of mAb dissected at thymic epithelial markers. To address the issue of the relationship between the thymic microenvironment and autoimmunity, we studied backcross (NZB x NZW) F1 x NZW mice in which 50% of offspring develop nephritis associated with proteinuria and anti-DNA antibodies. We reasoned that if thymic abnormalities are associated with development of disease, the correlation of abnormalities with lupus-like disease in individual backcross mice will form the foundation for identification of the mechanisms involved. In parallel, we directed a genetic linkage analysis, using markers previously shown to be linked to nephritis and IgG autoantibody production, to determine if such loci were similarly associated with microenvironmental changes. Our data demonstrate that all (NZB x NZW) F1 x NZW backcross mice with disease have microenvironmental defects. Although the microenvironmental defects are not sufficient for development of autoimmune disease, the severity of thymic abnormalities correlates with titers of IgG autoantibodies to DNA and with proteinuria. Consistent with past studies of (NZB x NZW) F1 x NZW mice, genetic markers on proximal chromosome 17 (near MHC) and distal chromosome 4 showed trends for linkage with nephritis. Although the markers chosen only covered about 10-15% of the genome, the results demonstrated trends for linkage with thymic medullary abnormalities for loci on distal chromosome 4 and distal chromosome 1. We believe it will be important to define the biochemical nature of the molecules recognized by these mAbs to understand the relationships between thymic architecture and immunopathology.     

癌灶CD4~+CD25~+和CD8~+ T细胞亚群与卵巢浆液性癌患者生存期相关

检测卵巢浆液性癌患者癌组织中CD4+CD25+及CD8+T细胞的数目,探讨其两种T细胞介导的免疫功能对疾病发展及预后的影响。免疫组织化学双标及单标的染色方法检测41例卵巢浆液性癌患者手术切除癌组织标本中CD4+CD25+和CD8+T细胞的数目。结果显示,癌灶中CD4+CD25+T淋巴细胞为(19.95±11.50)个/10HPF,CD8+T淋巴细胞为(43.46±16.69)个/10HPF。生存分析发现高CD4+CD25+T细胞组患者总生存期较低CD4+CD25+T细胞组缩短,差异有显著性(P<0.05);而高CD8+T细胞组患者总生存期与低CD8+T细胞组相比延长,且差异有显著性(P<0.05),此外两种T细胞数目与患者年龄、病理分级、临床分期、腹水细胞学及淋巴结转移等临床病理因素均无关(P>0.05)。结果表明,卵巢浆液性癌中高CD4+CD25+T细胞提示患者预后不良,可能与CD4+CD25+T细胞介导的免疫抑制导致肿瘤免疫逃逸有关;癌组织中高CD8+T细胞提示患者预后较好,两种T细胞对卵巢浆液性癌预后的评估有重要的价值,同时可以通过阻断CD4+CD25+T细胞的免疫抑制作用改善卵巢浆液性癌患者的预后,为卵巢癌治疗提供靶目标。     

Effect of murine thymic epithelial cell line (MTEC1) on the functional expression of CD4(+)CD8(-) thymocyte subgroups

To determine the effect of thymic stromal cells on the functional maturation of CD4 single-positive (SP) thymocytes, the functional status of isolated CD4 SP thymocyte subgroups was investigated by means of cell proliferation and cytokine production in response to concanavalin A (Con A) prior and after co-culturing with a murine thymic epithelial cell line (MTEC1). Mouse medullary CD4 SP thymocytes were phenotypically divided into seven discrete subgroups predicted to reflect the maturation pathway from newly emerging CD4 SP thymocytes to terminally differentiated cells. For functional analysis, six major subgroups (6C10(+)CD69(+), 6C10(-)CD69(+), 6C10(-)CD69(-)3G11(+)Qa-2(-), 6C10(-)CD69(-)3G11(+)Qa-2(+), 6C10(-)CD69(-)3G11(-)Qa-2(-) and 6C10(-)CD69(-)3G11(-)Qa-2(+)) cells were isolated and their functional status in response to Con A stimulation assessed. A functional hierarchy is revealed among these subgroups, consistent with their phenotypic maturation status, which may imply that these cells undergo a functional maturation process within thymic medulla. The function of cytokine production by CD4 SP thymocytes is acquired in a stepwise manner from a low to high level and characterized by T(h)0-type cytokines in the main stream of differentiation pathway. However, a minor subgroup that appeared at the late stage as 3G11(-)6C10(-) cells was biased to produce T(h)2-type cytokines. Nevertheless, the functional capacity of the final two Qa-2(+) subgroups of CD4 SP thymocytes was still significantly lower than that of spleen CD4(+) T cells. After co-cultivation with MTEC1 cells, four subgroups of TCRalphabeta(+)CD4(+)CD8(-) thymocytes exhibited significantly higher levels of proliferation capability and modulation in cytokine production capability. However, co-culturing with MTEC1 cells did not change the pattern of T(h)0- or T(h)2-like cytokine production by respectively medullary CD4 SP thymocyte subgroups nor could MTEC1 induce CD4 SP thymocytes to secrete T(h)1-type cytokines. The results suggest that MTEC1 can regulate the functional status of these thymocyte subgroups.     

Serial study of CD5+ and CD5- B cell subpopulations in 335 HIV seropositive patients.

B cell subpopulations, as defined by double-labelling techniques with CD5 and CD19 monoclonal antibodies (MoAbs), were serially studied in 335 HIV-1 seropositive patients. At the time of the first consultation, no important modifications in either CD5+ or CD5- subpopulations were observed, whatever the stage of the disease. However, in 18 out of the 335 patients (5.37%), a sharp increase in B cells exceeding 20% and 300/mm3 was observed. This increase in B cells was mainly accounted for CD5-CD19+ B cell subpopulations and was associated with: (i) evolution of the disease, since only four patients presented it at their first consultation (one lymphadenopathy-associated syndrome (LAS) and three AIDS); (ii) advanced stages of disease since, at the time of B cell augmentation, two patients were staged as LAS, four as ARC and 12 as AIDS; (iii) a high incidence of non-Hodgkin's lymphomas (NHL) since three out of the 18 patients presented a histologically confirmed NHL and three others a clinical pattern compatible with this diagnosis. However, in three patients with B hyperlymphocytosis, polymerase chain reaction (PCR) studies of immunoglobulin gene rearrangement revealed the existence of a polyclonal expansion of B cells. These results justify inclusion of a pan-B cell marker in routine phenotypic studies of HIV-infected individuals, as well as the search for NHL among patients presenting this abnormality.     

Association of a tetraspanin CD9 with CD5 on the T cell surface: role of particular transmembrane domains in the association

CD9 is a member of the tetraspanin superfamily which is characterized by four transmembrane (TM) domains and associates with other surface molecules. This tetraspanin was recently found to be expressed on mature T cells. Here, we investigated which molecules associate with CD9 on T cells and which CD9 domains are required for the association. Immunoprecipitation of T cell lysates with anti-CD9 mAb followed by immunoblotting with mAb against various T cell molecules showed the association of CD9 with CD3, CD4, CD5, CD2, CD29 and CD44. Because association with CD5 was most prominent, we determined the role of CD9 TM or extracellular (EC) domains in the association with CD5. CD9 mutant genes lacking each domain were constructed and introduced into EL4 thymoma cells deficient in CD9 but expressing CD5. Among various types of stable EL4 transfectants, EL4 transfected with the mutant gene lacking TM domains (TM2/TM3) between two EC domains expressed a small amount of the relevant protein without showing association with CD5. CD9(-)CD5(-) monkey COS-7 cells transfected with this mutant gene and the CD5 gene expressed both transfected gene products, but the association of these was not detected. EL4 cells transfected with a CD9/CD81 chimera gene (the CD9 gene containing TM2/TM3 of CD81) expressed the chimeric protein on the cell surface and showed association with CD5. These results suggest an essential role of particular CD9 TM domains in the surface expression of the CD9 molecule as well as the association with CD5.     

Clonality analysis performed using human androgen receptor assay in a rare case of undifferentiated thymic carcinoma coexisting with type AB thymoma

We report a very rare case of combined thymic carcinomas: undifferentiated thymic carcinoma coexisting with type AB thymoma. The precise mechanism underlying the coexistence of these tumors remains unknown. Therefore, we used clonality analysis to ascertain whether the two tumors were clonally related. A 63‐year‐old woman with thyroid cancer visited our hospital. Chest computed tomography also revealed an anterior mediastinal tumor. The patient was treated with total thyroidectomy and surgery for mediastinal tumors together with left upper lobe partial resection. The mediastinal tumor was pathologically diagnosed as undifferentiated thymic carcinoma coexisting with type AB thymoma. Multiple pulmonary metastases were detected in the patient and stage IV disease was diagnosed. The tumor was treatment‐resistant, and the patient received fourth‐line chemotherapy. We conducted clonality analysis using an improved human androgen receptor gene‐amplification assay that involves random X‐chromosome inactivation through methylation, followed by methylated gene‐specific PCR amplification after sample DNA digestion with HpaII, a methylation‐sensitive restriction enzyme. Clonality analysis demonstrated identical X‐chromosome inactivation in cells present in both thymoma and thymic carcinoma areas, and thus revealed clonal proliferation. The two lesions in the patient might have arisen through the transformation of a preexisting thymoma into a more malignant lesion.     

Comparative study of CD34, alpha-SMA and h-caldesmon expression in the stroma of gynaecomastia and male breast carcinoma

AIMS: To address the fibroblastic/myofibroblastic nature of stroma in gynaecomastia and in male breast carcinoma, the expression of CD34, alpha-smooth muscle actin (SMA) and h-caldesmon in the stromal cells was investigated by immunohistochemistry. METHODS AND RESULTS: Representative archival paraffin blocks were collected from male patients with gynaecomastia (32 cases) and mammary carcinoma (24 cases) between 1984 and 2004 and CD34, alpha-SMA and h-caldesmon were assessed immunohistochemically using a streptavidin-biotin method. Thirty cases of gynaecomastia showed a CD34+, alpha-SMA- and h-caldesmon- immunophenotype with different CD34 staining intensity in the various histological subtypes. Positivity for alpha-SMA and negativity for CD34 and h-caldesmon was found in a case of florid gynaecomastia relating to reactive fibrosis due to previous surgical intervention. Acquisition of alpha-SMA expression by stromal fibroblasts but absence of CD34 staining was identified in 22 cases of male breast carcinoma. CONCLUSIONS: The immunophenotype of periductal connective tissue stroma in gynaecomastia appears to parallel the phenotype of normal breast stroma. In male breast carcinoma the stromal cell immunophenotype is similar to that of its female counterpart showing myofibroblastic differentiation. However alpha-SMA+ and CD34- are not specific to malignancy because such findings are also encountered in reactive fibrosis.     

Characterization of porcine CD5 and CD5+ B cells

Evidence strongly supports a role for the lymphocyte transmembrane glycoprotein CD5 in intracellular signalling events, whereby antigen-dependent growth and differentiation signals are augmented. Apart from its role in activation-related signalling, CD5 has been regarded as a possible B cell lineage marker differentiating subsets, CD5⁺ B cells (also termed B1 cells) and conventional B cells (or B2 cells). To extend these investigations to the study of pigs, porcine B cells were examined for evidence of CD5 expression. The influence of cellular activation on CD5 expression and CD5's role in signal transduction events and lymphocyte proliferation were examined. Using an anti-porcine CD5 MoAb (b53b7), porcine B cells were shown to be heterogeneous for CD5 expression. As in other species, B lymphocyte CD5 expression is low (dull), while IgM is high (bright). Ten to 30% of pig blood B lymphocytes are CD5⁺, with the highest frequency in neonates. Anti-CD5 antibody treatment was sufficient to induce rapid but transient calcium ion flux in porcine peripheral blood lymphocytes (PBL). CD5 expression increased on PBL following treatment with phorbol myristate acetate (PMA), lipopolysaccharide (LPS), or immobilized anti-IgM. LPS, PMA, and concanavalin A (Con A) but not anti-CD5, anti-IgM, or combinations of these antibodies induced lymphocyte ³H-thymidine uptake. CD5⁺ B cells are a common constituent of porcine circulating lymphocytes and resemble B1 cells of mice, man and other species in CD5 expression, frequency and lymphoid organ distribution. Porcine CD5, like CD5 in other species, mediates signal transduction, leading to changes in intracellular calcium concentration, but this signal alone is insufficient to promote cell division. A subset of porcine B cells up-regulates CD5 expression following phorbol ester activation.