Mechanisms of physiologic B cell responses and B cell hyperactivity in systemic lupus erythematosus

B-cell biology

In recent years, our understanding of B-cell biology and the roles of B cells in normal immune responses and autoimmunity has increased dramatically. We no longer think of B cells simply as antibody factories. It is clear that these diverse and exquisitely regulated cells may contribute in a multitude of ways to immune responses. Animal models, clinical trials of biologic agents, and the ever expanding field of molecular biology have made great contributions to our current knowledge. With this improved understanding, we are afforded the opportunity to consider numerous potential therapeutic targets for treating autoimmune disease. As this growing science evolves, we can expect to see the advent of new therapies and new hope for patients who are afflicted with these disorders.

Increased levels of tissue plasminogen activator with a low plasminogen activator inhibitor-1 in a patient with postoperative bleeding

Recurrent postoperative bleeding in a previously healthy man with a moderate prolongation of PT and PTT which did not correct with vigorous fresh-frozen plasma infusion was identified as a unique abnormality of the fibrinolytic system: a combination of an excess of tissue plasminogen activator (t-PA) at 300% of normal activity accompanied by a concurrent deficiency of plasminogen activator inhibitor-1 activity (PAI-1) of 10-20% of normal activity. This observation underscores the potentially important regulatory role of PAI-1 in the degree to which clinical manifestations of bleeding tendencies may occur in patients in whom an excess amount of t-PA is expressed.

Detection of circulating Fc epsilon R2/CD23+ monocytes in patients with rheumatic diseases

Recently, in vitro studies have demonstrated that expression of Fc epsilon R2/CD23 on normal monocytes can be specifically induced by IL-4. In order to investigate the interaction of IL-4 and monocytes in rheumatic diseases, flow cytometry studies were performed. Elevated numbers of circulating Fc epsilon R2/CD23+ monocytes were detected in patients with progressive systemic sclerosis (PSS) as compared with controls. In addition, supernatants derived from phytohaemagglutinin-stimulated peripheral blood mononuclear cells of PSS patients contained high activity to induce Fc epsilon R2/CD23 on CD14+ monocytes. An increased frequency of Fc epsilon R2/CD23+ monocytes was also observed in rheumatoid arthritis, and sequential studies in patients with systemic lupus erythematosus showed a close relationship between Fc epsilon R2/CD23+ monocytes and disease activity. It is suggested that IL-4 has an important role in the pathogenesis of PSS by activating monocytes, and might also contribute to monocyte activation in other rheumatic diseases.

Decreased expression of interleukin-2 binding molecules (p70/75) in T cells from patients with systemic lupus erythematosus

Using radiolabeled interleukin-2 (IL-2), affinity cross-linking and binding assays revealed that the expression of intermediate-affinity IL-2 binding molecules (p70/75) on freshly prepared T cells from patients with systemic lupus erythematosus (SLE) was significantly decreased in comparison with that in normal subjects. The proliferative response of T cells to high doses of IL-2 was also reduced in patients with SLE. The decreased expression of p70/75 reflects the hyporesponsiveness to IL-2 of T cells in patients with SLE.

Theoretical and practical aspects of B-cell activation: murine and human systems

We have reviewed observations which were made during studies of murine and human B-cell responses in vitro. One currently faces difficulties in drawing any clear schema as to which external signals elicit which responses (activation, proliferation, differentiation) in B cells. However, the most potent antigen-dependent or polyclonal B-cell responses in vitro occur when, in addition to various cytokines, accessory cells, serum etc., the cultures contain either a) intact T-helper cells which enter into cell-to-cell contact with B cells, or b) some B-cell "mitogen" (T-independent antigen). Murine B cells activated with LPS and anti-Ig antibodies represent a model for the study of IL2 receptor expression and function. LPS does not act on human B cells. Certain mutant EL-4 thymoma cells are potent activators of murine and human B cells via a direct cell-to-cell interaction. The majority of human B cells can be induced to proliferate and generate a few hundred antibody-secreting cells each in the presence of such thymoma cells and a mixture of cytokines. From a practical point of view, this observation should be useful in a variety of investigations such as the analysis of the human B-cell specificity repertoire.