Long-lived lymphocytes include lipopolysaccharide-reactive B cells

Cell proliferation in the bone marrow, thymus and spleen of mice studied by continuous, in vivo bromodeoxycytidine labelling and flow cytometric analysis

We have applied the technique of labelling dividing cells with bromodeoxyuridine (BrdUrd) in combination with in vivo continuous labelling, propidium iodide (PI) staining for DNA content, and flow cytometric analysis, for the determination of cell proliferation in bone marrow, thymus and spleen of mice. The percentage of BrdUrd labelled cells increased as a function of exposure time in a tissue specific manner for each of the three tissues. Thymus and bone marrow had cell populations which exhibited different kinetics for the accumulation of label: (1) those that cycled and became labelled within 2-3 days (88% in 2 days for bone marrow, 84% in 3 days for thymus); (2) those that cycled during the remainder of the 6 day infusion period (11% of bone marrow and 13% of thymus cells); and (3) those that did not cycle during the 6 day period studied (less than 2% of bone marrow and 3% of thymus cells). In contrast, the spleen exhibited a slower, constant accumulation of labelled cells. After six days of infusion a large proportion of spleen cells (50%) had not become labelled. These results suggest that a larger proportion of spleen cells are long lived than indicated by other methods. We also have found the period of labelling with BrdUrd extended several days beyond the period of infusion. This method will be very useful in studying perturbations of cell populations induced in mice exposed to toxic agents.

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.

The majority of "natural" immunoglobulin-secreting cells are short-lived and the progeny of cycling lymphocytes

Treatment of mice with hydroxyurea to selectively kill all cycling cells has been used to study population dynamics and life expectancy of "natural" immunoglobulin-secreting cells in the bone marrow and spleen of nonimmunized animals. The results show that 50 to 90% of those cells are eliminated 2 to 3 days after one cycle of hydroxyurea administration, demonstrating their recent origin from cycling precursors. Using a protocol of long-term hydroxyurea treatment which abrogates cell production from the cycling precursors compartments, it was shown that "natural" immunoglobulin-secreting cells have a very short half renewal time, in the range of 15-60 h.

Effects of antimitotic treatment on haematopoietic tissues in mice

The antimitotic drug hydroxyurea was injected into mice twice daily for one, two, three or four consecutive days, and its effects assessed in bone marrow (BM) and spleen (SP). The number of nucleated cells decreased with the number of injections, reaching 25% of normal control values in SP and less than 10% in BM after four cycles. In contrast, the number of red blood cells (RBC) also dropped in SP, but increased in BM. Histological and morphometric studies showed that the increased number of RBC in BM could result from two major modifications in the marrow tissue: an expansion of sinusoids and a possible extravasation of cells from the marrow vasculature.

Long-lived B cells: mitogen reactivity as a tool for studying their life-spans

A population of noncycling small lymphocytes has been selectively enriched in mice by treatment with hydroxyurea (HU) over 4 consecutive days. This cell population, present in the spleen of HU-treated mice, contained a high frequency of lipopolysaccharide (LPS)-reactive cells and this could be used as a functional marker to follow their persistence after transfer to LPS-nonresponder recipient mice. The results reported here show that cells selected after a long-term HU treatment have considerably less decay, after transfer in recipient mice, than normal spleen cells. Furthermore, they demonstrate that HU-selected B cells survive in the recipient mice for periods up to 3 months, and are, consequently, true long-lived lymphocytes.

Population dynamics of bone marrow B lymphocytes

The dynamic concepts of lymphocyte populations which heralded the era of modern cellular immunobiology have been generally substantiated by recent studies and are still being correlated with functional properties. B lineage cells in the bone marrow are dynamically heterogeneous: A large majority are newly-formed, rapidly renewed cells, continuously produced from precursor cells within the bone marrow and disseminated during a terminal maturation phase via the blood stream. These cells develop low densities of sIgM in the extravascular bone marrow parenchyma and may undergo some further maturation within bone marrow sinusoids. The rate of production of bone marrow B cells appears to depend partly on the total load of exogenous agents to which the individual is exposed. Bone marrow lymphocyte production maintains a population of rapidly renewed virgin B cells in the peripheral lymphoid tissues. A small proportion of these cells apparently may be selected to enter a long-lived pool of B cells if suitably activated. By continuously creating novel clonotypes this process potentially can anticipate new antigen challenges and allow the immune system to build up a repertoire of antigen specificities most appropriate to the individual's changing environment throughout life. A minority of B lymphocytes in the bone marrow comprises slowly renewed, long-lived cells which enter and leave the bone marrow parenchyma as a selective part of the recirculating lymphocyte pool in the blood stream. Their role in the bone marrow is unknown. They include antigen-specific B memory cells, yet these are not activated within the bone marrow itself. No regulatory role has yet been directly demonstrated. Recently activated B cells enter from the spleen after secondary antigenic stimulation to develop into antibody-producing cells within the bone marrow. In assessing the significance of any phenotypically or functionally distinct B cell subset in the bone marrow, a basic consideration is to assign the subset to one of the foregoing dynamic categories. Within a given category cells may represent one stage in a time sequence of development. The bone marrow also produces lymphocytes of as yet uncertain lineage and contains selected subsets of T cells. The roles of these cells in cytotoxic, regulatory, or other events remain to be elucidated.

Lymphocyte population kinetics in the mouse

In a normal dynamic equilibrium, at least half of the peripheral T-cell pool is constituted by lymphocytes which have divided 24-48 h previously, and are therefore rapidly renewed. The renewal of peripheral T cells occurs partly by influx of cells from the thymus and, more importantly, by cell division at the periphery. The cyclic pattern of decay observed for T cells after HU treatment suggests the presence of progenitor-descendent relationships within the peripheral T-cell pool. Peripheral progenitors must contain both cycling and non-cycling cells to account for cell recovery after HU administration in ATx mice. T-cell production at the periphery involves both organized (spleen or lymph nodes) as well as non-organized lymphoid tissue (GALT). The latter may in fact provide the major contribution. Expansion of mature T lymphocytes contributes to clonal persistence at the periphery and to the choice of T-cell repertoires. The importance of post-thymic selection of T-cell repertoires is suggested by the considerable expansion potential revealed by peripheral T cells.