The studies describe alterations after hypophysectomy in the proportion of the type-1 and type-2 fibres in rat skeletal muscles, and the effects of replacement treatment with pituitary human (h) GH. Cytochemical analysis of myosin ATPase, succinate dehydrogenase and lactate dehydrogenase activities in sections of rat hind limb muscles were used as markers of fibre type and revealed that hypophysectomy reduced the proportion of type-1 fibres by 50% in soleus and in extensor digitorum longus muscles. This reduction in the proportion of type-1 fibres was accompanied by the appearance of transitional fibres (type 2C/1B). Following seven daily injections of hGH (60 mIU/day) to hypophysectomized rats, the proportion of type-1 fibres in both soleus and in extensor digitorum longus was increased with a concomitant reduction in the number of transitional fibres. After 11 days of treatment, all these transitional fibres had reverted back to type-1 fibres. Only hGH was observed to elicit this effect; injections of other pituitary hormones had no effect on the proportions of these transitional fibres. These alterations in fibre type occurred more rapidly than the changes reported after prolonged electrical stimulation of muscle or following extended exercise. These findings suggest that hypophysectomy and GH injection can result in a rapid alteration in the fibre composition of skeletal muscle, which may have important implications in terms of the resistance to fatigue and speed of contraction of the muscle.

Ig are multifunctional molecules with distinct properties assigned to individual domains. To assess the importance of IgM domain assembly in B cell development we generated two transgenic mouse lines with truncated muH chains by homologous integration of the neomycin resistance gene (neo(r)) into exons C(mu)1 and C(mu)2. Upon DNA rearrangement shortened muH chain transcripts, V(H)-D-J(H)-C(mu)3-C(mu)4, are produced independent of the transcriptional orientation and termination signals provided by neo(r). The truncated muH chain of approximately 52 kDa associates non-covalently with the L chain to form a monovalent HL heterodimer. Surface IgM is assembled into a defective BCR complex which has lost important signalling capacity. In immunizations with T-dependent and T-independent antigens, specific IgM antibodies cannot be detected, whilst IgG responses remain normal. B cell development in the bone marrow is characterized by an increase in early B cells, but a decrease of B220(+) cells from the stage when muH chain rearrangement is completed. The peritoneal lymphocyte population has elevated levels of CD5(+) B cells and their expansion may be the result of a negative feedback mechanism. The results show that antigenic stimulation is compromised by truncated monovalent IgM and that this deficit in stimulation leads to reduced levels of conventional B-2 lymphocytes, but dramatically increased levels of B-1 cells.

The majority of the human Ig heavy chain (IgH) constant (C) region locus has been cloned and mapped. An exception is the region between Cδ and Cγ3, which is unstable and may be a recombination hot spot. We isolated a pBAC clone (pHuIgH3′δ-γ3) that established a 52-kb distance between Cδ and Cγ3. Sequence analysis identified a high number of repeat elements, explaining the instability of the region, and an unusually large accumulation of transcription factor-binding motifs, for both lymphocyte-specific and ubiquitous transcription activators (IKAROS, E47, Oct-1, USF, Myc/Max), and for factors that may repress transcription (ΔEF1, Gfi-1, E4BP4, C/EBPβ). Functional analysis in reporter gene assays revealed the importance of the Cδ-Cγ3 interval in lymphocyte differentiation and identified independent regions capable of either enhancement or silencing of reporter gene expression and interaction with the IgH intron enhancer Eμ. In transgenic mice, carrying a construct that links the β-globin reporter to the novel δ-γ3 intron enhancer (Eδ-γ3), transgene transcription is exclusively found in bone marrow B cells from the early stage when IgH rearrangement is initiated up to the successful completion of H and L locus recombination, resulting in Ab expression. These findings suggest that the Cδ-Cγ3 interval exerts regulatory control on Ig gene activation and expression during early lymphoid development.

Here we compare human monoclonal antibody (MAb) production from mouse strains that carry disruptions of their endogenous mouse IgH/Ig loci and harbor human IgM + Ig (BAB) or human IgM + Ig + Ig transloci (BAB,). We found that whereas both strains proved effective for the isolation of antigen-specific IgM antibodies, many of the IgM MAbs elicited from BAB comprise human chains that are associated with mouse chains. In contrast, BAB, mice gave rise to fully functional, polymeric human IgM antibodies comprising both human IgH and human IgL chains. Therefore, the inclusion of a human Ig translocus (in addition to the human IgH + Ig transloci) not only diminishes problems of endogenous mouse Ig expression but also provides a strain of mice that yields fully human MAbs to a wide range of antigens, as witnessed by the isolation of MAbs to human blood cells, tumor cell lines, and an immunoglobulin idiotype.

Here we compare human monoclonal antibody (MAb) production from mouse strains that carry disruptions of their endogenous mouse IgH/Igκ loci and harbor human IgM + Igκ (BABκ) or human IgM + Igκ + Igλ transloci (BABκ,λ). We found that whereas both strains proved effective for the isolation of antigen-specific IgM antibodies, many of the IgM MAbs elicited from BABκ comprise human µ chains that are associated with mouse λ chains. In contrast, BABκ,λ mice gave rise to fully functional, polymeric human IgM antibodies comprising both human IgH and human IgL chains. Therefore, the inclusion of a human Igλ translocus (in addition to the human IgH + Igκ transloci) not only diminishes problems of endogenous mouse Igλ expression but also provides a strain of mice that yields fully human MAbs to a wide range of antigens, as witnessed by the isolation of MAbs to human blood cells, tumor cell lines, and an immunoglobulin idiotype.