Luciana Forni

IgD Receptors of Lymphoid Cells

While present in normal human serum in very low amounts and undetectable in sera of non-human primates as well as of mice, IgD is found on the surface of the majority of B lymphocytes in all the above mentioned species. Lymphocytes which carry IgD on their membrane also have IgM. The two molecules are present in relative amounts that can be very different in different cells. Both IgM and IgD of a single cell are the actual product of the cell itself. They have the same light chain and, more importantly, the same combining site and idiotype. IgD/IgM bearing lymphocytes are the majority of all B lymphocytes in spleen, lymph nodes and Peyers patches, whereas in bone marrow they account for half of the immunoglobulin positive cells. Although the percentage of double IgD/IgM cells is very similar in different tissues, the total amount of IgD, as well as the relative amounts of IgD and IgM as detected by biochemical methods varies. In fact, lymph nodes, and even more Peyers patches are much richer than spleen in cells having levels of IgD higher than those of IgM; conversely, in the bone marrow, all the positive cells have very low levels of IgD. In ontogeny, as in evolution, IgD appears after IgM: in human foetuses IgD bearing cells are not detectable before 13 weeks of gestation, and in the mouse they appear only after birth. IgD receptors seem to disappear from B cells which undergo maturation to secretion, as indicated by the fact that only a proportion of IgM secreting plasma cells show membrane IgD. IgD is never found on the membrane of IgG-containing cells, and also lymphocytes bearing simultaneously IgD and IgG are very rare, and it might well be that for these cells, the double expression for short periods of time does not actually correspond to simultaneous synthesis.

Strain differences in the postnatal development of the mouse splenic lymphoid system

Four strains of mice, CBA/J, BALB/c, C57BL/6J and B10.BR, were studied for cellular composition of the developing spleen in postnatal life. Considerable strain differences were found in the absolute numbers of splenic lymphoid cells at various ages, the frequencies of B and T cells, the L3T4/Lyt-2 ratio and the time of appearance of IgD and class II antigen on B cells. These observations are discussed in view of strain differences described for the acquisition of immune responsiveness and for susceptibility to tolerance induction in neonates. Finally, it was found that the expression of I-E antigen is delayed compared to that of I-A in ontogeny and possibly during B-cell differentiation as well.

The Vβ8 gene family is preferentially used by naturally activated T cells

Receptors encoded by the Vβ8 gene family detected by the monoclonal antibody F23.1 are expressed among ‘naturally’ activated T cells in normal spleen at frequencies significantly higher than in the total CD4+ and CD8+ cell populations. The positive selection of these clones into ‘natural’ T‐cell activity could be the reason for the high frequencies of cells expressing Vβ8 genes. This phenotype is strain‐dependent.

A phenotypic and functional analysis of long-lived B and T lymphocytes

The lymphocyte composition of spleen, lymph nodes, bone marrow, and thymus of mice submitted to hydroxyurea treatments for four consecutive days was studied. The treatment selects for small lymphocyte populations that represent between 4 and 20% of control numbers in the various organs. Spleen and bone marrow contain the same B cell population with a low IgM, high IgD, low I-E phenotype, which respond to LPS at control clonal frequencies. The T cell compartment is equally depleted, and the lymphocytes remaining contain frequencies of clonable cells in response to mitogens and IL-2 that are comparable to those detected in normal spleen cells. Overall, the results suggest that only a minor fraction of all lymphocytes in a normal young adult mouse have life spans longer than 4 days.