Transcription factors regulating B cell fate in the germinal centre

Diversification of the antibody repertoire is essential for the normal operation of the vertebrate adaptive immune system. Following antigen encounter, B cells are activated, proliferate rapidly and undergo two diversification events; somatic hypermutation (followed by selection), which enhances the affinity of the antibody for its cognate antigen, and class‐switch recombination, which alters the effector functions of the antibody to adapt the response to the challenge faced. B cells must then differentiate into antibody‐secreting plasma cells or long‐lived memory B cells. These activities take place in specialized immunological environments called germinal centres, usually located in the secondary lymphoid organs. To complete the germinal centre activities successfully, a B cell adopts a transcriptional programme that allows it to migrate to specific sites within the germinal centre, proliferate, modify its DNA recombination and repair pathways, alter its apoptotic potential and finally undergo terminal differentiation. To co‐ordinate these processes, B cells employ a number of ‘master regulator’ transcription factors which mediate wholesale transcriptomic changes. These master transcription factors are mutually antagonistic and form a complex regulatory network to maintain distinct gene expression programs. Within this network, multiple points of positive and negative feedback ensure the expression of the ‘master regulators’, augmented by a number of ‘secondary’ factors that reinforce these networks and sense the progress of the immune response. In this review we will discuss the different activities B cells must undertake to mount a successful T cell‐dependent immune response and describe how a regulatory network of transcription factors controls these processes

Cisplatin pharmacokinetics in elderly patients

The treatment of elderly patients with cancer may be difficult because of the narrow therapeutic index of antineoplastic drugs, the decline in the performance of organs and functions, and frequent comorbidity. In these patients, therapeutic drug monitoring may be useful in optimizing chemotherapy. Six patients older than 70 years with a variety of solid tumors received a total of 21 cycles of cisplatin (DDP)-based chemotherapy program (DDP dose, 50\u201365 mg/m2). Total and ultrafilterable platinum (Pt) were determined in plasma and urine during the first cycle of the therapy by means of atomic absorption spectroscopy. Pharmacokinetic parameters were analyzed with use of a two-compartment open model. The treatment was generally well tolerated. The most important side effects were a significant increase in serum creatinine level (from 0.98 to 1.23 mg/dl) and a decrease in creatinine clearance (from 44.4 to 38.9 ml/min) in comparison with pretreatment values. The mean decrease in hemoglobin levels was slight. The values of the main pharmacokinetic parameters of total Pt agreed well with the data obtained with other adult patients. Total and ultrafilterable Pt had a short distribution phase (t1/2\u3b1 = 0.35 and 0.54 h, respectively) followed by a prolonged elimination phase (t,1/2\u3b2 = 63.08 and 58.91 h, respectively). A reduced ability to clear ultrafilterable Pt (ClT = 123.52 ml h-1 kg-1) was evident and, as a result, the area under the curve increased (15.47 mg h L-1). The limited number of patients and the concomitant use of other agents prevent any firm conclusions. However, the documented impaired clearance of ultrafilterable Pt suggests that it may be safer in the elderly to use DDP at lower doses than in the adult and that this is associated with optimal drug exposure. \ua9 1994 Raven press, Ltd., New York

miR-29b directly targets activation-induced cytidine deaminase in human B cells and can limit its inappropriate expression in naïve B cells.

Class-switch recombination (CSR) is an essential B cell process that alters the isotype of antibody produced by the B cell, tailoring the immune response to the nature of the invading pathogen. CSR requires the activity of the mutagenic enzyme AID (encoded by AICDA) to generate chromosomal lesions within the immunoglobulin genes that initiate the class switching recombination event. These AID-mediated mutations also participate in somatic-hypermutation of the immunoglobulin variable region, driving affinity maturation. As such, AID poses a significant oncogenic threat if it functions outside of the immunoglobulin locus. We found that expression of the microRNA, miR-29b, was repressed in B cells isolated from tonsil tissue, relative to circulating naïve B cells. Further investigation revealed that miR-29b was able to directly initiate the degradation of AID mRNA. Enforced overexpression of miR-29b in human B cells precipitated a reduction in overall AID protein and a corresponding diminution in CSR to IgE. Given miR-29b's ability to potently target AID, a mutagenic molecule that can initiate chromosomal translocations and "off-target" mutations, we propose that miR-29b acts to silence premature AID expression in naïve B cells, thus reducing the likelihood of inappropriate and potentially dangerous deamination activity