The blood/brain barrier prevents the passive diffusion of proteins and metabolites from cerebral blood vessels into tissue spaces around neuronal and glial cells. To provide nutrients for these cells, transport mechanisms must exist and indeed have been demonstrated for metabolites. We now show that monoclonal antibodies against rat and human transferrin receptors label blood capillaries in the brain but not in other tissues. In the rat this labelling occurs after injection of antibody into the blood, thus the receptors seem to be accessible at the endothelial surface. It is possible that transferrin receptors are expressed on these cells to allow transport of transferrin (and thus iron) into brain tissues.

The transport of calcium ions (Ca(2+)) to the cytosol is essential for immunoreceptor signaling, regulating lymphocyte differentiation, activation, and effector function. Increases in cytosolic-free Ca(2+) concentrations are thought to be mediated through two interconnected and complementary mechanisms: the release of endoplasmic reticulum Ca(2+) "stores" and "store-operated" Ca(2+) entry via plasma membrane channels. However, the identity of molecular components conducting Ca(2+) currents within developing and mature T cells is unclear. Here, we have demonstrated that the L-type "voltage-dependent" Ca(2+) channel Ca(V)1.4 plays a cell-intrinsic role in the function, development, and survival of naive T cells. Plasma membrane Ca(V)1.4 was found to be essential for modulation of intracellular Ca(2+) stores and T cell receptor (TCR)-induced rises in cytosolic-free Ca(2+), impacting activation of Ras-extracellular signal-regulated kinase (ERK) and nuclear factor of activated T cells (NFAT) pathways. Collectively, these studies revealed that Ca(V)1.4 functions in controlling naive T cell homeostasis and antigen-driven T cell immune responses.

All the specific radiosensitive cells required for the induction and expression of the primary antibody response to sheep erythrocytes in the rat are normally present among the small lymphocytes in thoracic duct lymph (1, 2). Experiments with radiation chimeras have now made it clear that the activity of thoracic duct cells in this response is due to the presence in rat lymph of two populations of small lymphocytes, one derived from the thymus (T lymphocytes) 1 and the other derived from the bone marrow without thymic influence (B lymphocytes).2 Thymus-marrow collaboration in the rat, first reported by Johnston and Wilson (3), is similar to that originally described in mice by Mitchell and Miller (4).The present work extends the earlier findings on radiation chimeras by demonstrating that thymus-and marrow-derived small ]ymphocytes are present in the thoracic duct lymph of normal rats, and establishes three criteria by which the two populations may be distinguished. These are: (a) a marked deficiency of uridine incorporation in vitro by B lymphocytes relative to T lymphocytes; (b) a small difference in the rate of sedimentation; (c) a physiological segregation after transfusion into the blood, into clearly distinct zones within the spleen, lymph nodes, and Peyer's patches, which define the areas through which the two cell populations recirculate. Materials and MelhodsAnimals.--Young adult male and female rats of the inbred HO and AO strains were used in this study.Collection from the Tlzoracic Dztct of Pure Marrow-Derived Lymphocy/es and Artificial Mixtures of ~Iarrow-Derived and T Lymphacyles.--Adult rats were thymectomized, irradiated with 1000 rads from a 6°Co source, and then injected intravenously with 10 7 bone marrow cells from 1 Abbreviations used in this" paper: B ]ymphocytes, peripheral bone marrow-derived small lymphocytes; DAB, Dulbecco's A q-B (buffered salt solution); D/E/FCS, fetal calf serum in Dulbecco's modified Eagle's medium; PBS, phosphate-buffered saline; T Iymphocytes, peripheral thymus-derived small ]ymphocytes.2 Scott, D. W., and J. C. Howard. Collaboration between thymus-derived and marrowderived thoracic duct lymphocytes in the hemolysin response of rat.

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The repopulation of the peripheral lymphoid compartment of lethally‐irradiated rats reconstituted with lymphopoietic stem cells was studied. Cell lineages were traced by using genetic markers of cell surface molecules: immunoglobulin allotype for B lymphocytes and peripheral T cell alloantigen for T lymphocytes. Provided the markers had been bred on to a genetic background congenic with the hosts, they conferred neither an advantage nor disadvantage in competition with unmarked cells. the degree of chimaerism measured the lymphopoietic activity of the restorative inoculum. the most potent activity was found in foetal liver and spleen; next was infant spleen and bone marrow; then young adult bone marrow. Peripheral lymphoid tissues showed very little activity and thymus cells were inert. This tissue distribution, the stability of the chimaerism and the substantial expansion of numbers from the injected cells all point to the assay measuring an early stem cell.The overlap of subpopulations of lymphocytes in the rat thoracic duct was studied. A method for the conjugation of fluorescein to antibodies while they are attached to immuno‐adsorbent affinity columns is also described.

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In rat bone marrow Thy-1 antigen is present on cells with membrane immunoglobulin and on precursors of peripheral B lymphocytesRat bone marrow cells carrying Thy-1 antigen were studied morphologically, and tested for their independence of the thymus and their relationship to the B lymphocyte lineage.Using a fluorescence-activated cell sorter t o separate Thy-l+ and Thy-1-fractions, it has been confirmed that up to 50 % of all nucleated bone marrow cells are Thy-l+, most of which have the morphology of small lymphocytes. Thy-1-cells were mainly neutrophils and erythroid. Thy-l+ cells were found also in the marrow of B rats (rats thymectomized as adults, irradiated and reconstituted with syngeneic bone marrow from thymectomized donors drained of recirculating lymphocytes), though at a lower frequency (roughly half) than of normal rats. In both normal and B rats about 1/4 of the Thy-l+cells also bore lymphocyte surface immunoglobulin (sIg), and these doubly labeled cells accounted for the majority (-2/3) of marrow cells carrying large amounts of sIg. Therefore, unlike mice, Thy-1 is not a marker of thymus-dependent lymphocytes in rats.The B precursor activity of marrow fractions was measured in a long-term reconstitution assay counting sIg+ cells in the thoracic duct of lethally irradiated recipients. Virtually all the precursors were in the Thy-l+ or sIg-fractions, and were barely detectable among Thy-1-or sIg+ cells. Thus, in the rat peripheral B lymphocytes descend from precursors bearing Thy-l antigen but lacking sIg.

The simplest form of the hypothesis postulating immunoglobulin (Ig) 1 as the receptor for antigen on small lymphocytes predicts that Ig should be detectable on the lymphocyte surface and that it should be synthesized by the cell bearing it, not acquired from an external source. These predictions have been confirmed for the category of small lymphocytes which carry relatively large amounts of Ig on their surface (2-6), which at least in the case of the rabbit (2-4) are not acquired passively. These lymphocytes are thymusindependent (B) cells, and function as precursors for antibody synthesis (6-8). The other category of small lymphocytes are thought to have differentiated via the thymus (T cells) and to be involved in cell-mediated immune responses (9). T cells carry much less surface Ig than B cells as detected by anti-Ig binding studies (7, 10): the Ig can only be revealed by the most sensitive methods which employ autoradiography after binding of [l~SI]anti-Ig (10-12). The key question of the origin of this Ig has not been previously answered.In an attempt to solve this problem we chose to examine rat thoracic duct lymphocytes (TDL) for two reasons. In the first place, analysis of the lymphocytes in rat TDL (12) has previously revealed a heavily labeling population which bound [125I]anti-Fab in the range 20,000-150,000 molecules per cell, and a lightly labeling one which bound 200-2,000 molecules. Secondly, the discovery of a light chain allotype in the rat (13-16) provides a genetic marker to identify the origin of the Ig irrespective of its class. In this paper we report the purification and radio-iodination of anti Ig-la allotype antibody and its use in quantitative studies with rat TDL. The binding of [125I]antiallotype was 1 Abbreviations used in this paper: antiallotype, purified PVG/c F(ab')2 anti-DA Ig-la allotype antibody; anti-Fab, purified rabbit F (ab')2 antirat Fab antibody; BSA, bovine serum albumin, isotonic and neutralized (1); BBSS, buffered balanced salt solution (1); DAB, Dulbecco's A + B solution; FBS, foetal bovine serum; DAB/FBS, DAB containing 2%