Lymphocyte traffic through lymph nodes and Peyer's patches of the rat: B- and T-cell-specific migration patterns within the tissue, and their dependence on splenic tissue

Early interaction of adeno-associated virus serotype 8 vector with the host immune system following intramuscular delivery results in weak but detectable lymphocyte and dendritic cell transduction

Following in vivo recombinant adeno-associated virus (rAAV)-based gene transfer, adaptive immune responses specific to the vector or the transgene product have emerged as a potential roadblock to successful clinical translation. The occurrence of such responses depends on several parameters, including the route of vector administration as well as the viral serotype and the genome configuration, either self-complementary (sc) or single-stranded (ss). These parameters influence rAAV vector-associated immunity by modulating the crosstalk between the vector and the host immune system, including vector ability to interact or even transduce lymphoid tissues in general and antigen-presenting cells (APCs) in particular. Little is known about immune cell populations that are targeted in vivo by rAAV vectors. Moreover, the transduction of dendritic cells is still controversial and not directly demonstrated. Here, we show that intramuscular administration of an sc rAAV8 vector in the mouse leads to a rapid distribution of viral genomes in the lymphoid tissues that is associated with transgene expression. Transduced cells were detected in follicular areas of the spleen and the draining lymph nodes. In addition to B and T lymphocytes, transduced professional APCs were detected although at very low frequency. In addition, viral genomes and transgene transcripts were also detected in these cell populations after ss rAAV8 vector administration. Although the functional significance of those observations needs further explorations, our results highlight an early and intricate interaction between the rAAV vector upon its in vivo delivery and the host immune system.

Random migration and signal integration promote rapid and robust T cell recruitment

To fight infections, rare T cells must quickly home to appropriate lymph nodes (LNs), and reliably localize the antigen (Ag) within them. The first challenge calls for rapid trafficking between LNs, whereas the second may require extensive search within each LN. Here we combine simulations and experimental data to investigate which features of random T cell migration within and between LNs allow meeting these two conflicting demands. Our model indicates that integrating signals from multiple random encounters with Ag-presenting cells permits reliable detection of even low-dose Ag, and predicts a kinetic feature of cognate T cell arrest in LNs that we confirm using intravital two-photon data. Furthermore, we obtain the most reliable retention if T cells transit through LNs stochastically, which may explain the long and widely distributed LN dwell times observed in vivo. Finally, we demonstrate that random migration, both between and within LNs, allows recruiting the majority of cognate precursors within a few days for various realistic infection scenarios. Thus, the combination of two-scale stochastic migration and signal integration is an efficient and robust strategy for T cell immune surveillance.

Blood Lymphocytes, Monocytes and NK Cells Modulate Their Expression of CD44, ICAM‐1, LFA‐1 and MHC Class II After Arrival Into Lymphoid Organs

Adhesion molecules expressed on surface membranes of lymphocytes and other leukocytes enable their entry into the lymphoid and other tissues. However, little is known about molecules that govern the transit of leukocytes through the parenchyma of lymphoid organs proper. We show that in comparison to blood leukocytes, the corresponding cells isolated from lymphoid organs, i.e., lymph nodes and spleen, have a significantly augmented expression of certain surface molecules. The helper and cytotoxic subsets of T cells, as well as B cells, display the increased expression of CD44, ICAM-1 and LFA-1, whereas B cells additionally show the augmented expression of MHC class II. In comparison with blood monocytes, splenic monocytes show the increased expression of ICAM-1 and MHC class II molecules. When compared with blood NK cells, splenic NK cells only show the increased expression of ICAM-1. The molecules, which we show to be up regulated upon the entry of leukocytes into lymphoid organs, could be involved in their retention within the tissue via cell-cell or cell-extracellular matrix interactions and in control of their transit through lymphoid tissues.

gamma-H2AX foci formation in peripheral blood lymphocytes of tumor patients after local radiotherapy to different sites of the body: Dependence on the dose-distribution, irradiated site and time from start of treatment

PURPOSE

To evaluate the relationship between an estimated integral total body radiation dose delivered and phosphorylated histone H2AX protein (gamma-H2AX) foci formation in peripheral blood lymphocytes of cancer patients.

MATERIAL AND METHODS

gamma-H2AX formation was quantified as the mean number of foci per lymphocyte (N(meanH2AX)) and the percentage of lymphocytes with > or =n foci. The integrated total body radiation dose was estimated from the dose volume histogram of patient's body corrected for the proportion of the body scanned by computed tomography for 3D treatment planning.

RESULTS

There was a strong linear correlation between the mean number of gamma-H2AX foci per lymphocyte in the peripheral blood sample and integrated total body radiation dose (r = 0.83, p < 0.0001). The slope of the relationship was dependent on the site of body irradiated. In comparison to chest irradiation with a slope of 8.7 +/- 0.8 foci Gy(-1), the slopes for brain, upper leg and pelvic sites were significantly shallower by -4.7, -4.3, and -3.8 Gy(-1), respectively (p < 0.0001), while the slope for upper abdomen irradiation was significantly larger by 9.1 +/- 2.6 Gy(-1) (p = 0.0007). There was a slight time effect since the start of radiotherapy on the slopes of the in vivo dose responses leading to shallower slopes (-1.5 +/- 0.7 Gy(-1), p = 0.03) later (> or =10 day) during radiotherapy. After in vitro irradiation, lymphocytes showed 10.41 +/- 0.12 foci per Gy with no evidence of inter-individual heterogeneity.

CONCLUSIONS

gamma-H2AX measurements in peripheral lymphocytes after local radiotherapy allow the estimation of the applied integral body dose. The site and time dependence have to be considered.

Lymphocyte numbers and subsets in the human blood. Do they mirror the situation in all organs?

Lymphocyte numbers in the blood are used to evaluate the immune status on a daily basis in medicine. Several studies have documented the normal ranges of lymphocytes and lymphocyte subsets in the peripheral blood. A variety of techniques and criteria have revealed clear differences between the lymphocyte subsets in childhood and adolescence. Race and gender are also variables for blood lymphocytes, and even environmental factors seem to influence the numbers of some lymphocyte populations. However, do all these variations in lymphocyte subsets in the peripheral blood mirror changes in the lymphocyte populations of the whole body, or is it just a result of different migratory habits of cells? The factors influencing the distribution of lymphocytes in the peripheral blood with regard to the different abilities of T and B cells to migrate to distinct lymphoid or non-lymphoid tissue are summarized. In addition it will be described how the removal of organs (e.g. thymus, spleen, liver) influences the distribution of lymphocytes in the blood. All these parameters should be considered not only in the clinical situation when the immune status of a patient is extrapolated from the lymphocyte numbers in the blood, but also when interpreting treatment effects in patients.

Natural killer cell behavior in lymph nodes revealed by static and real-time imaging

Natural killer (NK) cells promote dendritic cell (DC) maturation and influence T cell differentiation in vitro. To better understand the nature of the putative interactions among these cells in vivo during the early phases of an adaptive immune response, we have used immunohistochemical analysis and dynamic intravital imaging to study NK cell localization and behavior in lymph nodes (LNs) in the steady state and shortly after infection with Leishmania major. In the LNs of naive mice, NK cells reside in the medulla and the paracortex, where they closely associate with DCs. In contrast to T cells, intravital microscopy revealed that NK cells in the superficial regions of LNs were slowly motile and maintained their interactions with DCs over extended times in the presence or absence of immune-activating signals. L. major induced NK cells to secrete interferon-γ and to be recruited to the paracortex, where concomitant CD4 T cell activation occurred. Therefore, NK cells form a reactive but low mobile network in a strategic area of the LN where they can receive inflammatory signals, interact with DCs, and regulate colocalized T cell responses.

Distribution of 99mTc-labeled lymphocytes in control and inflamed rats

We studied the in vivo fate of CD8(+) lymphocytes, of naive (CD8(+)CD45RC(bright)) or memory (CD8(+)CD45RC(dim)) phenotype, injected in syngeneic rats, after their sorting and labeling with [(99m)Tc] HM-PAO. By using the scintigrafic method we showed that memory CD8(+) lymphocytes were able to recirculate into liver and lungs. The same method was also successfully used to in vivo study the homing of total blood lymphocytes obtained from inflamed rats.

The fate of effector T cells in vivo is determined during activation and differs for CD4+ and CD8+ cells

Effector T cells generated in the mesenteric lymph nodes (mLN) are known to accumulate in mLN and the tissue drained by them after circulating in the blood. Their accumulation is due less to preferential entry into mLN but more to preferential proliferation within mLN. The factors regulating the proliferation of effector T cells in vivo are unclear, and it is unknown whether they are different for CD4(+) and CD8(+) effector T cells. Rat T cells from mLN or peripheral lymph nodes (pLN) were stimulated polyclonally via the TCR and CD28 and injected i.v. into congenic recipients. Using three-color flow cytometry and immunohistochemistry, they were identified in mLN, pLN, and blood over time, and proliferation was determined by measuring bromodeoxyuridine incorporation. Only effector mLN T cells showed a significantly increased proliferation rate after entry into mLN compared with that in pLN (2.4 +/- 1.8% vs 0.8 +/- 0.4%). Proliferation among the injected cells was higher when they had contact with dendritic cells within mLN (9.0 +/- 4.3%) than when they did not (4.1 +/- 2.1%). Furthermore, effector mLN T cells which were observed 56 days after injection maintained the capacity for preferential proliferation within mLN. Interestingly, CD4(+) effector mLN T cells proliferated at a higher rate (4.8 +/- 0.7%), remaining in mLN, whereas CD8(+) effector mLN T cells proliferated at a lower rate (3.3 +/- 1.0%) and were able to leave the mLN into the blood. Elucidating the factors regulating the proliferation of effector T cells in vivo will help to modify their distribution for therapeutic purposes.

Lymphocyte function-associated antigen-1 and intercellular adhesion molecule-1 expression on B-cell subsets and the effects of splenectomy-experimental studies

There is an abundance of data dealing with recirculation of T cells in the rats, but relatively little is known about the traffic of B cells. The adhesion molecules expressed on the surface membrane are of great significance for recirculation of lymphocytes. However, very little is known about the expression of various adhesion molecules on B-cell subsets. Here we show that in normal rats various adhesion molecules are differentially expressed on B-cell subsets and that the level of their expression changes after the entry of B lymphocytes from the blood into the lymphoid tissues. In splenectomized rats, the surface expression of LFA-1 and ICAM-1 is selectively reduced on B-cell subsets in blood and lymph node, which is accompanied by a selective increase in the number of all B-cell subsets in the blood. The decreased surface expression of adhesion molecules results in faster migration of B lymphocytes through lymph nodes with subsequent accumulation of these cells in the blood.

Splenectomy of rats selectively reduces lymphocyte function-associated antigen 1 and intercellular adhesion molecule 1 expression on B-cell subsets in blood and lymph nodes

Splenectomy increases the number of B cells in the blood of humans and animals. It is unknown whether this is due to changes in migration, proliferation, or both. The numbers of naïve (IgD(+)IgM(+)), memory (IgD(-)IgM(high)), newly formed (IgM(high)CD90(high)), early recirculating follicular (IgM(low)CD90(high)), recirculating follicular (IgM(low)CD90(-)), and marginal zone (IgM(high)CD90(-)) phenotype B cells were determined in control and splenectomized rats by flow cytometry. All subsets increased significantly in the blood after splenectomy. Because surface molecules are involved in the regulation of migration and proliferation, their expression (lymphocyte function-associated antigen 1 [LFA-1], intercellular adhesion molecule 1 (ICAM-1), L-selectin, alpha4-integrins, CD44, major histocompatability complex class II, interleukin 2 receptor-alpha chain) was determined on B- and T-cell subsets of both groups. B cells, but not T cells, showed a significantly reduced LFA-1 and ICAM-1 expression in blood and lymph nodes, whereas the expression of the other surface molecules analyzed remained unchanged. The down-regulation of these molecules did not influence the adherence of B cells to high endothelial venules in vitro. In vivo, however, ICAM-1(low)-expressing B cells migrated significantly faster through lymph nodes (ICAM-1(low) 41 +/- 5 hours versus ICAM-1(high) 58 +/- 3 hours), whereas proliferation of B cells in bone marrow, lymph node, and blood remained unchanged. Thus, the presence of one organ is necessary for appropriate expression of LFA-1 and ICAM-1 on B cells in other, distant organs. The more rapid transit of ICAM-1(low) B cells through lymph nodes may be responsible for the increased B-cell number in the blood after splenectomy.

Regeneration of implanted splenic tissue in the rat: re-innervation is host age-dependent and necessary for tissue development

The loss of spleen may lead to fatal bacterial infections. To prevent this, splenic autotransplantation has been performed in humans and experimental animals. However, there is still controversy about the protective function of this procedure. Since innervation plays an important role in splenic function, we investigated whether splenic regenerates are re-innervated, and whether this depends on the donor and host age. Splenic tissue (30 mg) was implanted into the greater omentum of either young (2 days) or old (12 months) rats, from either young or old syngeneic animals. After 3 months of regeneration, the weight of the regenerates was determined, PGP+ nerve fibers were revealed by immunohistology, and subdivided into nerve fibers of sympathetic (TH+, NPY+) or sensory (SP+, CGRP+) origin. In addition, proliferating (Ki-67 proliferation antigen+) and apoptotic cells (TUNEL technique+) were likewise investigated. No innervation of splenic regenerates was observed after implantation into old hosts, correlating with poorly developed splenic compartments. In contrast, almost normal re-innervation occurred in young hosts after implantation of both young and old splenic tissue. These regenerates showed well-developed splenic compartments and a normal number and tissue distribution of proliferating and apoptotic cells. However, after the implantation of young tissue, the final size of splenic regenerates was three times larger (140 +/- 30 vs. 40 +/- 10 mg). Thus, re-innervation of splenic implants is necessary for their subsequent development. It is determined by host age, whereas the final size of the splenic regenerates is regulated by donor age-dependent factors. This model is useful for studying both the process leading to initial innervation and the consequences of this innervation.

CD4+ T cells of both the naive and the memory phenotype enter rat lymph nodes and Peyer's patches via high endothelial venules: within the tissue their migratory behavior differs

It is thought that naive T cells predominantly enter lymphoid organs such as lymph nodes (LN) and Peyer's patches (PP) via high endothelial venules (HEV), whereas memory T cells migrate mainly into non-lymphoid organs. However, direct evidence for the existence of these distinct migration pathways in vivo is incomplete, and nothing is known about their migration through the different compartments of lymphoid organs. Such knowledge would be of considerable interest for understanding T cell memory in vivo. In the present study we separated naive and memory CD4+ T cells from the rat thoracic duct according to the expression of the high and low molecular weight isoforms of CD45R, respectively. At various time points after injection into congenic animals, these cells were identified by quantitative immunohistology in HEV, and T and B cell areas of different LN and PP. Three major findings emerged. First, both naive and memory CD4+ T cells enter lymphoid organs via the HEV in comparable numbers. Second, naive and memory CD4+ T cells migrate into the B cell area, although in small numbers and continuously enter established germinal centers (GC) with a bias for memory CD4+ T cells. Third, memory CD4+ T cells migrate faster through the T cell area of lymphoid organs than naive CD4+ T cells. Thus, our study shows that memory CD4+ T cells are not excluded from the HEV route. In addition, "memory" might depend in part on the ability of T cells to specifically enter the B cell area and GC and to screen large quantities of lymphoid tissues in a short time.

The lung as a source and a target organ for T- and B-lymphocytes

In lung transplantation, a substantial number of donor leukocytes are transferred from the donor to the recipient by the graft. Using a rat model, it was analyzed in this study to what extent leukocytes leave the lung, to which phenotype they belong, and to which organs they migrate. The model used was the orthotopic transplantation of the left lung of LEW.7B(RT7b) rats into LEW(RT7a) recipients. Lung allografts are not rejected in this strain combination, which differs only in the RT7 system, a genetic polymorphism of CD45. Using the RT7b marker (monoclonal antibody His41), the distribution of donor leukocytes passively transferred with the graft was studied by immunohistology 2 wk after transplantation. At this time, 2.9 +/- 0.1% (n = 6) of the peripheral blood leukocytes in the recipients were derived from the donor lung. The donor cell population detected in the blood consisted of T cells (59 +/- 4%), B cells (5.1 +/- 0.2%) and a surprisingly high fraction of natural killer (NK) cells (36 +/- 3%). No monocytes or granulocytes were found. In lymph nodes, spleen and thymus donor-derived T- and B-cells could be shown in typical T- and B-areas, respectively. Donor-derived leukocytes were found in the liver and the skin. In the tissue and the bronchoalveolar lavage (BAL) of the host lung, predominantly T cells were found. Furthermore, in the donor tissue and BAL more than 70% of T- and B-cells were host type, demonstrating that the donor lung had been repopulated to a great extent by host lymphocytes. This supports the relevance of BAL as a diagnostic tool in lung diseases. Thus, the lung is an immunologically important site, releasing lymphocytes which migrate to other organs and also attracting many lymphocytes from the circulation.

The fate of activated T cells migrating through the body: rescue from apoptosis in the tissue of origin

After activation within a lymphoid tissue, T lymphocytes enter the blood, where they circulate and then re-enter many organs. However, they predominantly end up in the tissue of origin, a phenomenon so far thought to be caused by organ-specific homing. We analyzed the fate of T cells from different sources stimulated via the T cell receptor and CD28 and then injected intravenously into rats. Our results showed that preferential proliferation and reduced apoptosis, rather than preferential immigration, were responsible for the accumulation of activated T cells in the tissue of origin, explaining how immune responses can spread from site to site but still be restricted to certain regions. Manipulating the life span of such cells might be a promising approach to influencing immune responses.

Cords, channels, corridors and conduits: critical architectural elements facilitating cell interactions in the lymph node cortex

The lymph node cortex is a critical site for encounter between recirculating T cells and their specific antigens. Due to its extreme plasticity, little is understood of the underlying functional unit of the lymph node cortex, the paracortical cord. The idealized paracortical cord (approximately 100 microns by 1000 microns) stretches from a medullary cord to the base of a B-cell follicle. In cross-section, a cord can be visualized as a set of nested cylinders consisting of spaces bounded by cells. The spaces are: i) the lumen of the high endothelial venule (HEV), ii) perivenular channels-narrow potential spaces (0.1 micron) tightly encircling the HEV, iii) corridors-broad spaces (10-15 microns) constituting the majority of the parenchyma, and iv) the cortical sinus. In addition to these spaces for cell traffic, the conduit (fifth space) is a special delivery system for the transit of soluble factors to the HEV and emigrating lymphocytes. The cellular barriers between these spaces are high endothelium, fibroblastic reticular cells, or sinus-lining cells. This review describes the spaces of the paracortical cord and their cellular boundaries, outlines the movement of cells and fluids through these spaces, and discusses how this anatomy affects the efficiency of surveillance by T cells.

Both activated and nonactivated leukocytes from the periphery continuously enter the thymic medulla of adult rats: phenotypes, sources and magnitude of traffic

Although the thymus is primarily noted for the export of T cells to the periphery, a small influx of cells has also been observed. It is still a matter of debate whether entry into the thymus depends on prior activation. The phenotypes, sources and degree of immigration are largely unknown. We monitored by quantitative immunohistochemistry the entry of cells from the periphery into the rat thymus in three experimental models. We injected i.v. recirculating, small, nonactivated CD4+ T cell subsets, often referred to as naive (CD45RC+) and memory or antigen-experienced (CD45RC-) cells, purified from thoracic duct lymph of allotype-marked donors, allotype-marked leukocytes released from spleen or lung transplants, or leukocytes labeled in the periphery for 12 weeks during the S-phase of the cell cycle by oral application of 5-bromo-2-deoxyuridine (BrdUrd). Early after i.v. injection (0.5 h), significantly more antigen-experienced (CD45RC-) CD4+ T cells entered the thymus, and by 24 h four times as many cells from the CD45RC- subset as from the CD45RC+ subset had entered the thymus and localized to the medulla. None of the thymic entrants expressed the interleukin (IL)-2 receptor. Following spleen transplantation approximately 40% of donor cells entering the thymic medulla were T cells and approximately 55% were B cells. In contrast, from a lung transplant, approximately 85% of peripheral immigrants were T cells and approximately 10% were B cells. After both procedures, a small number of NK cells and monocytes/macrophages were found among the immigrants (< 5%). Rats were fed BrdUrd continuously for 12 weeks, a procedure which labeled approximately 30% of peripheral lymphocytes but not cortical thymocytes. BrdUrd-labeled cells were localized almost exclusively to the thymic medulla and represented approximately 10% of medullary cells. Of the thymic immigrants approximately 50% were T cells, approximately 30% were B cells (including approximately 15% IgD+ cells), approximately 15% were NK cells and the remainder (approximately 5%) were monocytes/macrophages. Only a quarter of BrdUrd-labeled cells expressed the IL-2 receptor. The thymus is continuously infiltrated by both activated and nonactivated leukocytes from the periphery, including T cells, B cells, NK cells and monocytes. These immigrants are supplied by lymphoid and nonlymphoid organs in a characteristic subset composition. Their entry is facilitated by prior antigen experience or activation. Thus, the participation of the thymic medulla in general leukocyte traffic suggests a mechanism by which the T cell repertoire could potentially be modulated by the peripheral tissues.