Glennda Smithson

Two models of murine B lymphopoiesis: a correlation

During B cell genesis in mouse bone marrow (BM), precursor B cells pass through a series of developmental stages that have been defined by changes in expression of various marker molecules. The use of dissimilar phenotypic criteria in different laboratories, however, has led to the formulation of disparate models of B lymphopoiesis not fully reconciled with one another. We have directly compared two such models, one based on expression of intracellular μ heavy chain of IgM (cμ) and terminal deoxynucleotidyl transferase (TdT), the other monitoring cell surface leukosialin (CD43), heat‐stable antigen (HSA; CD24) and the ectopeptidase BP‐1. Each model uses cell surface B220 glycoprotein (CD45RA) to denote the B cell lineage. We have examined the cellular composition of four sorted BM fractions by immunofluorescent labeling of CD43, HSA and BP‐1, using immunofluorescence microscopy of cytocentrifuged fractions to quantitate precursor B cell populations expressing either cμ or TdT. The results reveal a range of B cell differentiation stages within individual sorted BM fractions, providing a cross‐reference between these two analytical methods and contributing to a unified model of B cell development in mouse BM.

Cell Interaction Molecules Utilized in Bone Marrow

Many aspects of blood cell formation can now be modeled in culture and rapid progress is being made in understanding how blood cell precursors interact with unique components of their environment. This brief review considers some cell interaction molecules that may be important for controlling the position of cells within, as well as their egress from, bone marrow.

BTNL8, a butyrophilin-like molecule that costimulates the primary immune response

The role of the B7 family molecules in the regulation of the immune response is well documented. A large body of experimental evidence indicates that costimulatory molecules such as B7-1, B7-2, B7-DC, B7-H1, B7-H2, B7-H3 and B7-H4 are critical for initiation, maintenance and down-regulation of the immune response. However the immunological function of butyrophilin (BTN)-like molecules, which are a part of the expanded B7 family, is not known. Here, we demonstrate that the extracellular portion of human BTNL8 can augment Ag-induced activation of T lymphocytes. BTNL8 has two alternatively spliced forms: B7-like and BTN-like. Both isoforms of BTNL8 were expressed concurrently in various human tissues. A putative BTNL8 receptor was detected only on resting T lymphocytes. Administration of BTNL8Ig fusion protein into mice promoted production of Ag-specific IgG during the primary, but not the secondary immune responses. BTNL8 may therefore play an essential role in priming of naïve T lymphocytes.

Reassessment of germline heavy chain transcripts from two murine VH families

While expression of functional heavy chain immunoglobulin mRNA requires rearrangement of variable (VH), diversity (D) and (JH) gene segments, these individual gene segments can be transcribed prior to their rearrangement. It has been proposed that the resulting germline, or sterile, transcripts play an important role in the rearrangement process because strong correlations between rearrangement frequency and sterile transcript levels have been observed in some studies. Murine VH genes have been grouped into families on the basis of coding sequence homology. VH families rearrange in a developmentally regulated manner, so that rearrangements of genes from several VH families are detected earlier than rearrangements of J558 family genes. Paradoxically, the only VH family for which sterile transcripts have been documented is the J558 family. We used RT-PCR analyses to ask whether sterile transcripts from other VH families could be detected in fetal liver samples prior to their rearrangement. While J558 family germline transcripts were easily detected, no sterile transcripts were observed from the S107 family. Our studies also revealed the ability of small quantities of degraded genomic DNA to nonspecifically prime cDNA synthesis, emphasizing the need for caution in interpreting RT-PCR data in which family-specific oligos are used for cDNA production. These results cast doubt on the idea that sterile transcripts are required for V(H)DJ(H) rearrangement.

Life/Death Decisions in B Lymphocyte Precursors

The bone marrow and thymus are remarkable factories for blood cells. Although impressively large numbers of cells of various types are produced, the output is carefully controlled. Lymphocytes that emerge from these organs are essential to life, but some are potentially capable of inducing autoimmune disease or malignancy. For that reason, intricate mechanisms have evolved for checking the maturing cells for quality and functional capability. Blood cells of most types can be made in other organs, as is particularly obvious during embryonic life, or when the marrow is ablated. However, this is not the case during normal adult circumstances, and it has long been a goal to determine what is special about central lymphoid tissues. Extraordinary progress has been made in discovering cytokines and cell interaction molecules produced in those sites, and we are beginning to understand how each delivers positive and negative signals for lymphocyte formation. However, additional interesting molecules are still being found, and new roles are being ascribed to previously known ones. The focus of this chapter will be on several classes of molecules expressed within the bone marrow environment. Their study may reveal mechanisms that control the movement of lymphocyte precursors within and from bone marrow, the rate of lymphocyte production, and how defective or potentially harmful cells are eliminated.