A model of lymphocyte recirculation

Human Immunodeficiency Virus Persistence in the Spleen: Opportunities for Pharmacologic Intervention

The persistence of HIV in the spleen, despite combination antiretroviral therapy, is not well understood. Sustained immune dysregulation and delayed immune recovery, in addition to immune cell exhaustion, may contribute to persistence of infection in the spleen. Eliminating HIV from this secondary lymphoid organ will require a thorough understanding of antiretroviral (ARV) pharmacology in the spleen, which has been minimally investigated. Low ARV exposure within the spleen may hinder the achievement of a functional or sterilizing cure if cells are not protected from HIV infection. In this study, we provide an overview of the anatomy and physiology of the spleen, review the evidence of the spleen as a site for persistence of HIV, discuss the consequences of persistence of HIV in the spleen, address challenges to eradicating HIV in the spleen, and examine opportunities for future curative efforts.

Predicted limited redistribution of T cells to secondary lymphoid tissue correlates with increased risk of haematological malignancies in asplenic patients

The spleen, a secondary lymphoid tissue (SLT), has an important role in generation of adaptive immune responses. Although splenectomy remains a common procedure, recent studies reported poor prognosis and increased risk of haematological malignancies in asplenic patients. The high baseline trafficking of T lymphocytes to splenic tissue suggests splenectomy may lead to loss of blood-borne malignant immunosurveillance that is not compensated for by the remaining SLT. To date, no quantitative analysis of the impact of splenectomy on the human T cell trafficking dynamics and tissue localisation has been reported. We developed a quantitative computational model that describes organ distribution and trafficking of human lymphocytes to explore the likely impact of splenectomy on immune cell distributions. In silico splenectomy resulted in an average reduction of T cell numbers in SLT by 35% (95%CI 0.12–0.97) and a comparatively lower, 9% (95%CI 0.17–1.43), mean decrease of T cell concentration in SLT. These results suggest that the surveillance capacity of the remaining SLT insufficiently compensates for the absence of the spleen. This may, in part, explain haematological malignancy risk in asplenic patients and raises the question of whether splenectomy has a clinically meaningful impact on patient responses to immunotherapy.

Modeling Radioimmune Response-Current Status and Perspectives

The combination of immune therapy with radiation offers an exciting and promising treatment modality in cancer therapy. It has been hypothesized that radiation induces damage signals within the tumor, making it more detectable for the immune system. In combination with inhibiting immune checkpoints an effective anti-tumor immune response may be established. This inversion from tumor immune evasion raises numerous questions to be solved to support an effective clinical implementation: These include the optimum immune drug and radiation dose time courses, the amount of damage and associated doses required to stimulate an immune response, and the impact of lymphocyte status and dynamics. Biophysical modeling can offer unique insights, providing quantitative information addressing these factors and highlighting mechanisms of action. In this work we review the existing modeling approaches of combined ‘radioimmune’ response, as well as associated fields of study. We propose modeling attempts that appear relevant for an effective and predictive model. We emphasize the importance of the time course of drug and dose delivery in view to the time course of the triggered biological processes. Special attention is also paid to the dose distribution to circulating blood lymphocytes and the effect this has on immune competence.

Measurement and Quantitative Characterization of Whole-Body Pharmacokinetics of Exogenously Administered T Cells in Mice

Here we have investigated whole-body pharmacokinetics (PK) of exogenously administered T cells in a mouse model of melanoma and have developed a physiologically based pharmacokinetic (PBPK) model to quantitatively characterize the data. T cells were isolated from the spleen of tumor-bearing mice, activated, and labeled with chromium-51 to facilitate the quantification. Labeled T cells were injected in the tumor-bearing mice, and PK was measured in 19 different tissues. It was found that T cells disappear from the blood rapidly after administration and accumulate in the tissues to various extents. Spleen, liver, lung, kidney, bone, and lymph nodes accounted for more than 90% of T cells in the body. The distribution of T cells in solid tumors was found to be very low, hovering below 1%ID/g (percent of injected dose per gram of tissue) during the entire study. However, this observation may differ for targeted TCR-T and CAR-T cells. Observed PK profiles also suggest that T-cell-based therapies may be more successful in treating cancers of the lymphatic system and bone marrow metastases compared to solid tumors. A PBPK model was developed to characterize the whole-body PK of T cells, which incorporated key processes such as extravasation, elimination, and recirculation of T cells via lymph flow. Retention factors were incorporated into the spleen, liver, and kidney compartment to adequately capture the PK profiles. The model was able to characterize observed PK profiles reasonably well, and parameters were estimated with good confidence. The PK data and PBPK model presented here provide unprecedented insight into the biodistribution of exogenously administered T cells.

Field size effects on the risk and severity of treatment-induced lymphopenia in patients undergoing radiation therapy for solid tumors

Purpose

Radiation-induced lymphopenia (RIL) is the result of direct toxicity to circulating lymphocytes as they traverse the irradiated field, occurs in 40% to 70% of patients who undergo conventional external beam radiation therapy, and is associated with worse outcomes in multiple solid tumors. As immunotherapy strategies evolve, a better understanding of radiation's effects on the immune system is needed in order to develop rational methods of combining RT with immunotherapy.

Methods and materials

This paper is a review of the available literature on the clinical significance and dosimetric predictors of radiation-induced toxicity to the immune system.

Results

An association between severe RIL and inferior survival has been described in multiple solid tumors, including glioma, lung cancer, and pancreatic cancer. RIL risk is correlated with field size, dose per fraction, and fraction number. SBRT and proton therapy techniques are associated with lower RIL risk.

Conclusions

The immune system should be considered an organ at risk during RT, and absolute lymphocyte count is an important biomarker of RT-induced immunotoxicity. Radiation dose and technique affect the risk and severity of RIL. Further research is needed to accurately characterize RT-induced immunotoxicity and develop strategies to prevent or mitigate this clinically significant side effect.

Increased T cell trafficking as adjunct therapy for HIV-1

Although antiretroviral drug therapy suppresses human immunodeficiency virus-type 1 (HIV-1) to undetectable levels in the blood of treated individuals, reservoirs of replication competent HIV-1 endure. Upon cessation of antiretroviral therapy, the reservoir usually allows outgrowth of virus and approaches to targeting the reservoir have had limited success. Ongoing cycles of viral replication in regions with low drug penetration contribute to this persistence. Here, we use a mathematical model to illustrate a new approach to eliminating the part of the reservoir attributable to persistent replication in drug sanctuaries. Reducing the residency time of CD4 T cells in drug sanctuaries renders ongoing replication unsustainable in those sanctuaries. We hypothesize that, in combination with antiretroviral drugs, a strategy to orchestrate CD4 T cell trafficking could contribute to a functional cure for HIV-1 infection.

Description of organ-specific phenotype, and functional characteristics of tissue resident lymphocytes from liver transplantation donor and research on immune tolerance mechanism of liver

Aim

Prior to transplantation, Donation after Cardiac Death (DCD) liver transplantation livers are perfused with preservation solution. Therefore, this provides an abundant source of human liver lymphocytes, as well as mesenteric lymph node and spleen for the study of lymphocyte subset diversity in the peripheral blood, lymph node, spleen and liver.

Methods

Lymphocyte subsets were isolated and purified from peripheral blood, lymph node, spleen and liver perfusion, the phenotypic and functional analysis of the tissue resident lymphocyte were performed by flow cytometry.

Results

In a direct comparison between blood, liver, lymph node and spleen cells from liver transplantation donors, the abundance of natural killer (NK) cells, CD3⁺CD56⁺NKT (NT) cells and CD8⁺ T cells in intrahapatic lymphocytes (IHL) did not match what was present in peripheral blood and other peripheral lymphoid organs. The activation state of peripheral blood-derived lymphocytes was significantly different from lymph node-, spleen- and liver-derived cells. Intriguingly, NK cells, CD4⁺ T cells, and CD8⁺ T cells from liver perfusion display more suppressive characteristics, that is, express and produce more anti-inflammatory cytokine interleukin (IL)-10, less inflammatory cytokine interferon (INF)-γ.

Conclusion

Our findings imply that different tissues entail resident lymphocyte subsets with a distinct phenotype and function considering the organ is well vascularized, particularly in liver. It is better to understand the mechanism of liver immune tolerance.

A Stochastic Model for CD4+ T Cell Proliferation and Dissemination Network in Primary Immune Response

The study of the initial phase of the adaptive immune response after first antigen encounter provides essential information on the magnitude and quality of the immune response. This phase is characterized by proliferation and dissemination of T cells in the lymphoid organs. Modeling and identifying the key features of this phenomenon may provide a useful tool for the analysis and prediction of the effects of immunization. This knowledge can be effectively exploited in vaccinology, where it is of interest to evaluate and compare the responses to different vaccine formulations. The objective of this paper is to construct a stochastic model based on branching process theory, for the dissemination network of antigen-specific CD4+ T cells. The devised model is validated on in vivo animal experimental data. The model presented has been applied to the vaccine immunization context making references to simple proliferation laws that take into account division, death and quiescence, but it can also be applied to any context where it is of interest to study the dynamic evolution of a population.

COMPUTATIONAL MODEL OF RECIRCULATING LYMPHOCYTES

Lymphocyte recirculation plays an important role in controlling the spread of both pathogenic infections and tumor-producing cancer cells in the human body. We present a mathematical and computational framework that allows investigation of recirculating lymphocytes and estimation of model parameters using a genetic algorithm. The framework allows estimating parameters using data obtained from experiments performed in laboratory studies of rats as well as clinical studies of human subjects. Our computational model allows improved understanding of these data. Mathematical models enable investigators to obtain a quantitative picture of immune system kinetics and diversity in human health and disease outcomes. Our data-driven systems biology and immunological modeling approach contributes to a growing understanding of the dynamics of lymphocyte recirculation.

Modeling the T cell immune response: a fascinating challenge

The immune system is designed to protect the organism from infection and to repair damaged tissue. An effective response requires recognition of the threat, the appropriate effector mechanism to clear the pathogen and a return to homeostasis with minimal damage to self-tissues. T cells play a central role in orchestrating the immune response at all stages of the response and have been the subject of intense study by both experimental immunologists and modelers. This review examines some of the more critical questions in T cell biology and describes the latest attempts to address those questions using approaches that combine mathematical modeling and experiments.

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.

Mathematical modeling reveals kinetics of lymphocyte recirculation in the whole organism

The kinetics of recirculation of naive lymphocytes in the body has important implications for the speed at which local infections are detected and controlled by immune responses. With a help of a novel mathematical model, we analyze experimental data on migration of 51Cr-labeled thoracic duct lymphocytes (TDLs) via major lymphoid and nonlymphoid tissues of rats in the absence of systemic antigenic stimulation. We show that at any point of time, 95% of lymphocytes in the blood travel via capillaries in the lung or sinusoids of the liver and only 5% migrate to secondary lymphoid tissues such as lymph nodes, Peyer's patches, or the spleen. Interestingly, our analysis suggests that lymphocytes travel via lung capillaries and liver sinusoids at an extremely rapid rate with the average residence time in these tissues being less than 1 minute. The model also predicts a relatively short average residence time of TDLs in the spleen (2.5 hours) and a longer average residence time of TDLs in major lymph nodes and Peyer's patches (10 hours). Surprisingly, we find that the average residence time of lymphocytes is similar in lymph nodes draining the skin (subcutaneous LNs) or the gut (mesenteric LNs) or in Peyer's patches. Applying our model to an additional dataset on lymphocyte migration via resting and antigen-stimulated lymph nodes we find that enlargement of antigen-stimulated lymph nodes occurs mainly due to increased entrance rate of TDLs into the nodes and not due to decreased exit rate as has been suggested in some studies. Taken together, our analysis for the first time provides a comprehensive, systems view of recirculation kinetics of thoracic duct lymphocytes in the whole organism.

Analysis of naïve lung CD4 T cells provides evidence of functional lung to lymph node migration

The proportion of CD4 T cells with phenotypic and functional properties of naïve cells out of total CD4 T cells is similar in the lung parenchyma and lymph nodes. On treatment with a sphingosine-1-phosphate agonist, the frequency of these cells falls precipitously, but with a delay of ∼14 h compared with blood CD4 T cells; neither anti-CD62L nor pertussis toxin prevents entry of naïve CD4 T cells into the lung. Based on treatment with anti-CD62L and the use of CCR7(-/-) cells, lung naïve CD4 T cells appear to migrate to the mediastinal lymph nodes along a CD62L-independent, CCR7-dependent pathway. Cells that have entered the node in this manner are competent to respond to antigen. Thus, a portion (approximately one-half) of naïve CD4 T cells appears to enter the mediastinal lymph nodes through a blood-to-lung-to-lymph node route.

Directional migration of recirculating lymphocytes through lymph nodes via random walks

Naive T lymphocytes exhibit extensive antigen-independent recirculation between blood and lymph nodes, where they may encounter dendritic cells carrying cognate antigen. We examine how long different T cells may spend in an individual lymph node by examining data from long term cannulation of blood and efferent lymphatics of a single lymph node in the sheep. We determine empirically the distribution of transit times of migrating T cells by applying the Least Absolute Shrinkage & Selection Operator (LASSO) or regularised S-LASSO to fit experimental data describing the proportion of labelled infused cells in blood and efferent lymphatics over time. The optimal inferred solution reveals a distribution with high variance and strong skew. The mode transit time is typically between 10 and 20 hours, but a significant number of cells spend more than 70 hours before exiting. We complement the empirical machine learning based approach by modelling lymphocyte passage through the lymph node insilico. On the basis of previous two photon analysis of lymphocyte movement, we optimised distributions which describe the transit times (first passage times) of discrete one dimensional and continuous (Brownian) three dimensional random walks with drift. The optimal fit is obtained when drift is small, i.e. the ratio of probabilities of migrating forward and backward within the node is close to one. These distributions are qualitatively similar to the inferred empirical distribution, with high variance and strong skew. In contrast, an optimised normal distribution of transit times (symmetrical around mean) fitted the data poorly. The results demonstrate that the rapid recirculation of lymphocytes observed at a macro level is compatible with predominantly randomised movement within lymph nodes, and significant probabilities of long transit times. We discuss how this pattern of migration may contribute to facilitating interactions between low frequency T cells and antigen presenting cells carrying cognate antigen.

The race for the prize: T-cell trafficking strategies for optimal surveillance

The initiation of T-cell responses requires rare precursors to locate a draining lymph node (dLN) and encounter dendritic cells (DCs) presenting peptide-major histocompatibility complexes (pMHCs). To locate this needle in the haystack rapidly, T cells face an optimization problem-what is the most efficient trafficking strategy for surveillance and recirculation through blood? Two extremes are scanning low numbers of DCs per node with frequent recirculation, or meticulous surveillance with infrequent recirculation. Naive T cells also require stimulation by self-pMHCs. To enable efficient location of both foreign and self, has evolution settled on an optimum time for T cells to spend surveying each lymph node? Using a data-driven mathematical model, we show the most efficient strategy for detecting antigen in a dLN depends on its abundance. Detection of low-density antigen is optimized with systemically slow transit. In contrast, at high densities or if dLN egress is restricted, rapid transit through other nodes is optimal. We argue that blood-lymph recirculation dynamics facilitate a trade-off, and are consistent with dominant roles for the very early detection of rare foreign antigens in a dLN, and the efficient accumulation of signals from systemically distributed self-antigens.

T-lymphocyte perturbation following large-scale apheresis and hematopoietic stem cell transplantation in HIV-infected individuals

Analysis and mathematical modeling of T-lymphocyte perturbation following administration of granulocyte colony stimulating factor (G-CSF) and two large-scale aphereses are reported. 74 HIV-1 positive antiretroviral-treated individuals were infused with gene- or sham-transduced CD34+ hematopoietic stem cells (HSC) in a Phase II clinical trial. T cell numbers were examined in four phases: 1) during steady state; 2) increases in peripheral blood (PB) following G-CSF administration; 3) depletion post-aphereses and 4) reconstitution post HSC infusion. The present analysis provides the first direct estimate of CD4+ T cell distribution and trafficking in HIV-infected individuals on stable HAART, indicating that CD4+ T lymphocytes in PB represent 5.5% of the pool of CD4+ T lymphocytes that traffic to PB.

Heparin-induced leukocytosis requires 6-O-sulfation and is caused by blockade of selectin- and CXCL12 protein-mediated leukocyte trafficking in mice

Leukocytosis refers to an increase in leukocyte count above the normal range in the blood and is a common laboratory finding in patients. In many cases, the mechanisms underlying leukocytosis are not known. In this study, we examined the effects, the structural determinants, and the underlying mechanisms of heparin-induced leukocytosis, a side effect occurring in 0.44% of patients receiving heparin. We observed that heparin induced both lymphocytosis and neutrophilia, and the effects required heparin to be 6-O-sulfated but did not require its anticoagulant activity. Cell mobilization studies revealed that the lymphocytosis was attributable to a combination of blockage of lymphocyte homing and the release of thymocytes from the thymus, whereas the neutrophilia was caused primarily by neutrophil release from the bone marrow and demargination in the vasculature. Mechanistic studies revealed that heparin inhibits L- and P-selectin, as well as the chemokine CXCL12, leading to leukocytosis. Heparin is known to require 6-O-sulfate to inhibit L- and P-selectin function, and in this study we observed that 6-O-sulfate is required for its interaction with CXCL12. We conclude that heparin-induced leukocytosis requires glucosamine 6-O-sulfation and is caused by blockade of L-selectin-, P-selectin-, and CXCL12-mediated leukocyte trafficking.

Rho GTPases: masters of T lymphocyte migration and activation

Rho GTPases are key signal transducer elements activated in T cells by both chemokine and antigen receptors. These two signalling pathways control the two main functions of T lymphocytes: motility and activation. Rho GTPases are thus crucial for the development of an adequate immune response. In this review, we mostly focus on the roles of RhoA, Rac1 and Cdc42 in T cells. We show their importance in phenomena such as adhesion, morphological polarization, migration and antigen recognition.

Defining the quantitative limits of intravital two-photon lymphocyte tracking

Two-photon microscopy has substantially advanced our understanding of cellular dynamics in the immune system. Cell migration can now be imaged in real time in the living animal. Strikingly, the migration of naive lymphocytes in secondary lymphoid tissue appears predominantly random. It is unclear, however, whether directed migration may escape detection in this random background. Using a combination of mathematical modeling and experimental data, we investigate the extent to which modern two-photon imaging can rule out biologically relevant directed migration. For naive T cells migrating in uninfected lymph nodes (LNs) at average 3D speeds of around 18 μm/min, we rule out uniform directed migration of more than 1.7 μm/min at the 95% confidence level, confirming that T cell migration is indeed mostly random on a timescale of minutes. To investigate whether this finding still holds for longer timescales, we use a 3D simulation of the naive T cell LN transit. A pure random walk predicts a transit time of around 16 h, which is in good agreement with experimental results. A directional bias of only 0.5 μm/min-less than 3% of the cell speed-would already accelerate the transit twofold. These results jointly strengthen the random walk analogy for naive T cell migration in LNs, but they also emphasize that very small deviations from random migration can still be important. Our methods are applicable to cells of any type and can be used to reanalyze existing datasets.

Naive CD4+ T lymphocytes circulate through lymphoid organs to interact with endogenous antigens and upregulate their function

Naive T lymphocytes recirculate through the lymph-vascular system and enter and exit lymphoid organs. Using mice expressing the photoconvertible fluorescence protein Kaede, we demonstrated that naive T cells seek to interact with endogenous Ags after migrating to the lymphoid organs. The interaction with endogenous Ags transiently induces CD69 expression on T cells, which prolongs retention in the lymphoid organs. Cells that fail to express CD69 or lose CD69 expression migrate to other lymphoid organs. Functionally, CD69(+)-naive CD4(+) T cells exhibit faster and greater cytokine production than do CD69(-) naive CD4(+) T cells. These results indicate that CD4(+) T cells continuously migrate to interact with endogenous Ags, and such an interaction plays an important role in the Ag reactivity of naive CD4(+) T cells.

Scaling aspects of lymphocyte trafficking

We consider the long lived pool of B and T cells that recirculate through blood, tissues and the lymphatic system of an animal with body mass M. We derive scaling rules (allometric relations) for: (1) the rate of production of mature lymphocytes, (2) the accumulation of lymphocytes in the tissues, (3) the flux of lymphocytes through the lymphatic system, (4) the number of lymph nodes, (5) the number of lymphocytes per clone within a lymph node, and (6) the total number of lymphocytes within a lymph node. Mass-dependent aspects of immune learning and of the immunological self are shown to be not very significant. Our treatment is somewhat heuristic and aims at combining immunological data with recent progress in biological scaling.

A hybrid agent-based approach for modeling microbiological systems

Models for systems biology commonly adopt Differential Equations or Agent-Based modeling approaches for simulating the processes as a whole. Models based on differential equations presuppose phenomenological intracellular behavioral mechanisms, while models based on Multi-Agent approach often use directly translated, and quantitatively less precise if-then logical rule constructs. We propose an extendible systems model based on a hybrid agent-based approach where biological cells are modeled as individuals (agents) while molecules are represented by quantities. This hybridization in entity representation entails a combined modeling strategy with agent-based behavioral rules and differential equations, thereby balancing the requirements of extendible model granularity with computational tractability. We demonstrate the efficacy of this approach with models of chemotaxis involving an assay of 10(3) cells and 1.2x10(6) molecules. The model produces cell migration patterns that are comparable to laboratory observations.

Monitoring cellular movement in vivo with photoconvertible fluorescence protein "Kaede" transgenic mice

Kaede is a photoconvertible fluorescence protein that changes from green to red upon exposure to violet light. The photoconversion of intracellular Kaede has no effect on cellular function. Using transgenic mice expressing the Kaede protein, we demonstrated that movement of cells with the photoconverted Kaede protein could be monitored from lymphoid organs to other tissues as well as from skin to the draining lymph node. Analysis of the kinetics of cellular movement revealed that each subset of cells in the lymph node, such as CD4(+) T, CD8(+) T, B, and dendritic cells, has a distinct migration pattern in vivo. Thus, the Kaede transgenic mouse system would be an ideal tool to monitor precise cellular movement in vivo at different stages of immune response to pathogens as well as in autoimmune diseases.

Analyzing the migration of labeled T cells in vivo: an essential approach with challenging features

T cells are involved in the pathogenesis of many diseases. To exert a pathological effect, T cells enter the tissues. We show that the determination of their entry site requires isolation of the respective T cell population, injection into genetically un-manipulated animals, and identification of the cells in vivo at various time points after injection. We indicate variables influencing in vivo migration experiments artificially, and outline how resulting problems can be either avoided or taken into account. Reviewing experiments performed according to the outlined criteria reveals two types of migration patterns for T cell subsets in vivo: 1). Naïve and memory T cells enter lymphoid and non-lymphoid organs in comparable numbers, but selectively accumulate in lymphoid tissues over time, 2). Effector T cells, too, enter lymphoid and non-lymphoid organs in comparable numbers. However, most of them die within 24 hours. Depending on the presence of cytokines, chemokines and extracellular matrix compounds they are able to survive, thereby preferentially accumulating in their target tissues. This information might help to understand the role of migration in the pathogenesis of T cell mediated diseases.

An application of linear output error modelling for studying lymphocyte migration in peripheral lymphoid tissues

Lymphocyte recirculation between lymphatic and blood vessels and migration through tissues are essential mechanisms underlying immunological surveillance. However, the kinetics of lymphocyte migration through lymphoid tissues remains poorly understood. The present study of lymphocyte migration, based on a sheep model and entailing the long term cannulation of blood vessels and lymphatic vessels efferent from lymph nodes, represents the first attempt to apply control engineering based models to overcome some of the experimental impediments to understanding the complex phenomena involved in lymphocyte migration. An output error model order (1,2,nk) was systematically selected under given criteria from four classes of Linear Time-Invariant Single-Input Single-Output, (LTI-SISO) systems to represent the peripheral lymph node system. The unit impulse responses were simulated under noise free conditions and their features were extracted to describe the dynamics of the system. The findings from this study revealed novel information about several aspects of the dynamics of lymphocyte migration.

Quantitative studies on the movement of fluid and lymphocytes through periodontal tissue and into the draining lymph

Chronic lymph drainage techniques in sheep have been used to map the pathways and to quantify the fluid and cell traffic through periodontal tissues. The continuous collection of cervical and prescapular lymph has demonstrated that 65% of labelled protein tracer injected into the periodontal tissues could be found in lymph over a period of 7.5 hours. Nearly 90% of the total radioactivity could be accounted for between the lymph and the tissue site. When silk was impregnated with radiolabelled albumin and a tooth ligated, the kinetics of the subsequent appearance of the tracer in lymph emphasized the ease with which macromolecules surrounding the teeth gain access to the lymphatics, regional lymph nodes, and immune apparatus. Animals were primed with BCG and then tuberculin (delayed hypersensitivity) lesions were simultaneously induced in the skin, bowel, and periodontium. When T cells were labelled with radioisotopes and their migration from blood to lymph measured, the periodontal tissue traffic pattern was distinct from the traffic pattern through DTH in the skin and also distinct from the pattern through the small intestine. This indicates that the lymphocyte traffic through the inflamed periodontium has unique features. This tissue specificity was not apparent when lesions were induced with TNFalpha. The static assessment of lymphocyte subsets within the tissues was also assessed with immunohistochemistry.

Dynamics of HIV infection: a cellular automata approach

We use a cellular automata model to study the evolution of human immunodeficiency virus (HIV) infection and the onset of acquired immunodeficiency syndrome (AIDS). The model takes into account the global features of the immune response to any pathogen, the fast mutation rate of the HIV, and a fair amount of spatial localization, which may occur in the lymph nodes. Our results reproduce the three-phase pattern observed in T cell and virus counts of infected patients, namely, the primary response, the clinical latency period, and the onset of AIDS. The dynamics of real experimental data is related to the transient behavior of our model and not to its steady state. We have also found that the infected cells organize themselves into spatial structures, which are responsible for the decrease on the concentration of uninfected cells, leading to AIDS.

Intralesional granulocyte-monocyte colony-stimulating factor followed by subcutaneous interleukin-2 in metastatic melanoma: a pilot study in elderly patients

AIM AND BACKGROUND

Recent data in the literature indicate that antigen-presenting cells (APC) are inactive in tumour tissue because of local immunosuppression. Tumour-infiltrating lymphocyte (TIL) signal activation transducing mechanisms are also seriously impaired. Administration of granulocyte macrophage-colony stimulating factor (GM-CSF) may lead to APC recovery and interleukin (IL)-2 may restore local TIL activation. Moreover, IL-2 increases the systemic lymphocyte population, an event that seems to correlate with a better prognosis.

STUDY DESIGN

The present phase I-II study was carried out to examine whether intralesional injection of GM-CSF followed by subcutaneous IL-2 would induce a clinical response in advanced, pretreated elderly melanoma patients.

METHODS

Sixteen patients over 60 years of age received intralesional GM-CSF (150 ng per lesion on day 1), generally divided between the two largest cutaneous lesions, followed by perilesional subcutaneous IL-2 (3,000,000 IU) for 5 days (days 3-7 inclusive) every 3 weeks.

RESULTS

Four clinical responses [two partial (PR) and two minimal (MR)] (25%), which also involved lesions that had not been directly treated, and nine cases of stable disease were observed. The response duration for PR and MR was 9, 4, 4 and 2.5 + months, respectively. Stable disease (56%) recorded in the nine patients was short-term (3-6 months). Three patients rapidly progressed after two, two and one therapy cycles, respectively. The patient who reached the best PR had a fairly high absolute lymphocyte count (1600-2400/mm3). The second one, who reached complete remission after subsequent locoregional chemotherapy and hyperthermia, however, had a low absolute lymphocyte count that had doubled by the end of treatment. Blood lymphocyte values in the other patients were too varied to allow any correlation with clinical response. Therapy was well tolerated and only mild fever was observed, with the exception of one patient who had grade 3 fever, with muscle pain and arthralgia.

CONCLUSIONS

Considering the very low toxicity observed, this treatment might be indicated in elderly patients for whom systemic therapy is no longer a viable option. Improved scheduling and timing could result from further studies.

Model of HIV-1 disease progression based on virus-induced lymph node homing and homing-induced apoptosis of CD4+ lymphocytes

Several proposed theories have described the progression of HIV infection. Even so, no concrete evidence supports any as comprehensive, including, for example, why the CD4+ T-cell counts fall from 1000/mm3 of blood to roughly 100/mm3 over an average 10-year period, whereas concomitant viral loads are relatively constant, increasing by several orders of magnitude in late-stage disease. Here, we develop and validate a theoretical model that altered lymphocyte circulation patterns between the lymph system and blood due to HIV-induced enhanced lymph-node homing and subsequent apoptosis of resting CD4+ T cells can explain many aspects of HIV-1 disease progression. These results lead to a recalculation of the CD4+ lymphocyte dynamics during highly active antiretroviral therapy, and also suggest new targets for therapy.

Modelling of peripheral lymphocyte migration: system identification approach

This is the first application of the prediction error method (PEM) of system identification to modelling lymphocyte migration through peripheral lymphoid tissue. The PEM was applied to the emergence of labelled lymphocytes from the efferent lymphatic of a lymph node following their intravenous administration. Advantages of PEM included the capacity to calculate the response to a unit impulse stimulus, unavailable to direct observation, and to allow for the return to the node of labelled cells that had already recirculated once. Calculation of the system delay (time between introduction of cells into the blood and their first appearance in lymph) indicated 4.67 +/- 1.05 h for the total lymphocyte population. The peak in efferent lymph occurred at 11.91 +/- 4.68 h, much earlier than previous reports, which were affected by cells that had already recirculated. While 75% of labelled cells had emerged in efferent lymph by 20.77 +/- 5.62 h, 86.38 +/- 29.44 h was required for 100% emergence. The considerable heterogeneity in migratory behaviour is likely to reflect frequency and duration of binding of lymphocytes by dendritic cells in paracortical cord corridors. It is proposed that differences in the speed with which lymphocytes pass along corridors depend on their functional status, in particular whether they are naïve or memory cells.

The traffic of resting lymphocytes through delayed hypersensitivity and chronic inflammatory lesions: a dynamic equilibrium

This essay is designed as a partial summary of the work of several students and colleagues from our laboratory. For the most part the experimental data have been published and this seminar represents an attempt to summarize, integrate and speculate on this work. In some situations the speculation is rather unrestrained and it is hoped that it will provoke discussion, controversy and better future experiments. Reference is made to the original articles and to recent reviews. In addition, since we have had the advantage of reading the other contributions to this volume, there is considerable reference to other chapters. We and others have argued in other publications that it is imperative to understand the normal physiological traffic of lymphocytes before one can adequately interpret data describing lymphocyte migration through pathological tissues. It has been useful to compare data derived from traffic through lymph nodes because there is considerable information on the individual lymph node with respect to blood-lymphocyte delivery and blood flow, prenodal input via peripheral lymphatics and, particularly in sheep, in the numbers and phenotypic analysis of the lymphocytes exiting lymph nodes in postnodal or efferent lymph (see Young, this volume). We consider the terms prenodal for afferent and post-nodal for efferent to be synonomous. In this volume, a significant contribution has been made by Cahill et al which describes the astonishing degree of lymphocyte traffic which occurs in fetal life prior to antigenic challenge. At the other extreme, in disease states, there is often profound activation of lymphocytes. This is most apparent in viral infections like HIV and SIV (Rosenberg et al, this volume) and also in the various models of diseases such as EAE (Hickey and Kulidjian et al, this volume). It is a central tenet of our paper that resting and activated lymphocyte migration need to be considered separately and that they are very different.

Immunosuppression without immunosuppression? How to be a tolerant individual in a dangerous world

The field of transplantation has developed based on two principles: allografts are rejected because they express foreign antigens, and the immune system must be suppressed to prevent rejection. Recently, in vitro and in vivo experimental evidence has accumulated that calls both of these beliefs into question. This article reviews an alternative approach to transplantation that focuses on tissue injury as the instigator of graft rejection and employs physiological mechanisms of tolerance to avoid graft loss. Methods that allow for defense against infectious microbes while at the same time allowing for graft survival are proposed. In particular, the rationale behind the use of anti-CD154 antibody treatment is highlighted. A model is introduced that takes into consideration the experimental successes seen with anti-CD154 therapies.

Vascular and non-vascular ligands for L-selectin

Ligands for L-selectin are expressed not only on vascular endothelial cells but also in the extravascular tissues. In this article, we summarize the current understanding of the "vascular" ligands for L-selectin. We also describe identification of "non-vascular" ligands for L-selectin and discuss their possible biological significance.

Role of the thymus in pediatric HIV-1 infection

Several lines of evidence suggest that HIV-1 is present in the thymus during HIV-1 infection. Precursors to mature CD4+ T lymphocytes develop in the thymus, which suggests that thymic infection may play a role in the CD4+ T-cell decline observed during the course of pediatric HIV-1 infection. We illustrate, through mathematical modeling, the potential effects of thymic infection on the course of pediatric AIDS disease progression. We find that infection in the thymus not only can supplement peripheral infection but can help explain the faster progression in pediatric cases, as well as the early and high viral burden.

The simulation of density-dependent effects in the recirculation of T lymphocytes

T-lymphocyte recirculation appears to be slower in nude or irradiated rats as compared with normal rats. A mathematical model of T-lymphocyte recirculation that incorporates interactions between T cells and dendritic cells in the lymphoid tissue is presented. It is shown that these interactions are able to explain the differences in recirculation times between normal and nude or irradiated rats, and also the time-scales seen in long-term thoracic duct cannulations.

Leukocyte removal filter-passed lymphocytes produce large amounts of interleukin-4 in immunotherapy for inflammatory bowel disease: role of bystander suppression

To determine immunosuppression by leukocytapheresis, we studied the immune profiles of filter-passed lymphocytes (FPLs) in patients with ulcerative colitis. These patients were treated with a leukocytapheresis (LCAP) filter of granulocyte apheresis (GCAP) column. Cytokine profiles or FPLs and peripheral blood mononuclear cells (PBMCs) were examined using interleukin (IL)-4 and interferon-gamma (IFN-gamma) immunoassay with lectin stimulation. The IL-4 production of LCAP FPLs was significantly higher than in prefilter PBMCs and GCAP column-passed lymphocytes (median: 540 pg/ml x 10(6) cells versus 4.7 and 2.9, respectively, p = 0.001). IL-4 production in PBMCs was increased after LCAP. IFN-gamma production was not increased in the FPLs. The IL-4 immune deviation was not shown in the patients treated with GCAP. LCAP FPLs showed increased CD4+DR- cells and decreases CD4+DR+ cells in comparison to PBMCs. LCAP FPLs could produce IL-4 and may lead to bystander suppression.

The molecular basis of leukocytosis

The function of any known gene is often found by DNA or protein homology scanning. Conversely, it is equally rewarding to search for the genetic basis behind a known function. Here, Ghislain Opdenakker and colleagues examine the known and possible novel genes and molecular events underlying the phenomenon of leukocytosis, one of the most common clinical manifestations of inflammatory problems.

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.