The regulation of self-tolerance and the role of inflammasome molecules.

Inflammasome molecules make up a family of receptors that typically function to initiate a proinflammatory response upon infection by microbial pathogens. Dysregulation of inflammasome activity has been linked to unwanted chronic inflammation, which has also been implicated in certain autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, type 1 diabetes, systemic lupus erythematosus, and related animal models. Classical inflammasome activation-dependent events have intrinsic and extrinsic effects on both innate and adaptive immune effectors, as well as resident cells in the target tissue, which all can contribute to an autoimmune response. Recently, inflammasome molecules have also been found to regulate the differentiation and function of immune effector cells independent of classical inflammasome-activated inflammation. These alternative functions for inflammasome molecules shape the nature of the adaptive immune response, that in turn can either promote or suppress the progression of autoimmunity. In this review we will summarize the roles of inflammasome molecules in regulating self-tolerance and the development of autoimmunity.

A Multitrait Locus Regulates Sarbecovirus Pathogenesis.

Infectious diseases have shaped the human population genetic structure, and genetic variation influences the susceptibility to many viral diseases. However, a variety of challenges have made the implementation of traditional human Genome-wide Association Studies (GWAS) approaches to study these infectious outcomes challenging. In contrast, mouse models of infectious diseases provide an experimental control and precision, which facilitates analyses and mechanistic studies of the role of genetic variation on infection. Here we use a genetic mapping cross between two distinct Collaborative Cross mouse strains with respect to severe acute respiratory syndrome coronavirus (SARS-CoV) disease outcomes. We find several loci control differential disease outcome for a variety of traits in the context of SARS-CoV infection. Importantly, we identify a locus on mouse chromosome 9 that shows conserved synteny with a human GWAS locus for SARS-CoV-2 severe disease. We follow-up and confirm a role for this locus, and identify two candidate genes, CCR9 and CXCR6, that both play a key role in regulating the severity of SARS-CoV, SARS-CoV-2, and a distantly related bat sarbecovirus disease outcomes. As such we provide a template for using experimental mouse crosses to identify and characterize multitrait loci that regulate pathogenic infectious outcomes across species. IMPORTANCE Host genetic variation is an important determinant that predicts disease outcomes following infection. In the setting of highly pathogenic coronavirus infections genetic determinants underlying host susceptibility and mortality remain unclear. To elucidate the role of host genetic variation on sarbecovirus pathogenesis and disease outcomes, we utilized the Collaborative Cross (CC) mouse genetic reference population as a model to identify susceptibility alleles to SARS-CoV and SARS-CoV-2 infections. Our findings reveal that a multitrait loci found in chromosome 9 is an important regulator of sarbecovirus pathogenesis in mice. Within this locus, we identified and validated CCR9 and CXCR6 as important regulators of host disease outcomes. Specifically, both CCR9 and CXCR6 are protective against severe SARS-CoV, SARS-CoV-2, and SARS-related HKU3 virus disease in mice. This chromosome 9 multitrait locus may be important to help identify genes that regulate coronavirus disease outcomes in humans.

A Multitrait Locus Regulates Sarbecovirus Pathogenesis.

Infectious diseases have shaped the human population genetic structure, and genetic variation influences the susceptibility to many viral diseases. However, a variety of challenges have made the implementation of traditional human Genome-wide Association Studies (GWAS) approaches to study these infectious outcomes challenging. In contrast, mouse models of infectious diseases provide an experimental control and precision, which facilitates analyses and mechanistic studies of the role of genetic variation on infection. Here we use a genetic mapping cross between two distinct Collaborative Cross mouse strains with respect to SARS-CoV disease outcomes. We find several loci control differential disease outcome for a variety of traits in the context of SARS-CoV infection. Importantly, we identify a locus on mouse Chromosome 9 that shows conserved synteny with a human GWAS locus for SARS-CoV-2 severe disease. We follow-up and confirm a role for this locus, and identify two candidate genes, CCR9 and CXCR6 that both play a key role in regulating the severity of SARS-CoV, SARS-CoV-2 and a distantly related bat sarbecovirus disease outcomes. As such we provide a template for using experimental mouse crosses to identify and characterize multitrait loci that regulate pathogenic infectious outcomes across species.

Immune Checkpoint Ligand Bioengineered Schwann Cells as Antigen-Specific Therapy for Experimental Autoimmune Encephalomyelitis.

Failure to establish immune tolerance leads to the development of autoimmune disease. The ability to regulate autoreactive T cells without inducing systemic immunosuppression represents a major challenge in the development of new strategies to treat autoimmune disease. Here, a translational method for bioengineering programmed death-ligand 1 (PD-L1)- and cluster of differentiation 86 (CD86)-functionalized mouse Schwann cells (SCs) to prevent and ameliorate multiple sclerosis (MS) in established mouse models of chronic and relapsing-remitting experimental autoimmune encephalomyelitis (EAE) is described. It is shown that the intravenous (i.v.) administration of immune checkpoint ligand functionalized mouse SCs modifies the course of disease and ameliorates EAE. Further, it is found that such bioengineered mouse SCs inhibit the differentiation of myelin-specific helper T cells into pathogenic T helper type-1 (Th 1) and type-17 (Th 17) cells, promote the development of tolerogenic myelin-specific regulatory T (Treg ) cells, and resolve inflammatory central nervous system microenvironments without inducing systemic immunosuppression.

In Vivo Bioengineering of Beta Cells with Immune Checkpoint Ligand as a Treatment for Early-Onset Type 1 Diabetes Mellitus.

Type 1 diabetes mellitus (T1DM) is an autoimmune disease caused by autoreactive T cells targeting the insulin-producing beta (ß) cells. Despite advances in insulin therapy, T1DM still leads to high morbidity and mortality in patients. A key focus of T1DM research has been to identify strategies that re-establish self-tolerance and suppress ongoing autoimmunity. Here, we describe a strategy that utilizes pretargeting and glycochemistry to bioengineer ß cells in situ to induce ß-cell-specific tolerance. We hypothesized that ß-cell-targeted Ac4ManNAz-encapsulated nanoparticles deliver and establish ß cells with high levels of surface reactive azide groups. We further theorized that administration of a dibenzylcyclooctyne (DBCO)-functionalized programmed death-ligand 1 immunoglobulin fusion protein (PD-L1-Ig) can be readily conjugated to the surface of native ß cells. Using nonobese diabetic (NOD) mice, we demonstrated that our strategy effectively and selectively conjugates PD-L1 onto ß cells through bioorthogonal stain-promoted azide-alkyne cycloaddition. We also showed that the in vivo functionalized ß cells simultaneously present islet-specific antigen and PD-L1 to the engaged T cells, reversing early onset T1DM by reducing IFN-gamma expressing cytotoxic toxic T cells and inducing antigen-specific tolerance.

Elicitation of broadly protective sarbecovirus immunity by receptor-binding domain nanoparticle vaccines.

Understanding vaccine-elicited protection against SARS-CoV-2 variants and other sarbecoviruses is key for guiding public health policies. We show that a clinical stage multivalent SARS-CoV-2 spike receptor-binding domain nanoparticle (RBD-NP) vaccine protects mice from SARS-CoV-2 challenge after a single immunization, indicating a potential dose-sparing strategy. We benchmarked serum neutralizing activity elicited by RBD-NPs in non-human primates against a lead prefusion-stabilized SARS-CoV-2 spike (HexaPro) using a panel of circulating mutants. Polyclonal antibodies elicited by both vaccines are similarly resilient to many RBD residue substitutions tested, although mutations at and surrounding position 484 have negative consequences for neutralization. Mosaic and cocktail nanoparticle immunogens displaying multiple sarbecovirus RBDs elicit broad neutralizing activity in mice and protect mice against SARS-CoV challenge even in the absence of SARS-CoV RBD in the vaccine. This study provides proof of principle that multivalent sarbecovirus RBD-NPs induce heterotypic protection and motivates advancing such broadly protective sarbecovirus vaccines to the clinic.

Coreceptor therapy has distinct short- and long-term tolerogenic effects intrinsic to autoreactive effector T cells.

Immunotherapies are needed in the clinic that effectively suppress ß cell autoimmunity and reestablish long-term self-tolerance in type 1 diabetes. We previously demonstrated that nondepleting anti-CD4 (αCD4) and αCD8α antibodies establish rapid and indefinite remission in recent-onset diabetic NOD mice. Diabetes reversal by coreceptor therapy (CoRT) is induced by suppression of pathogenic effector T cells (Teffs) and the selective egress of T cells from the pancreatic lymph nodes and islets that remain free of infiltration in the long term. Here, we defined CoRT-induced events regulating early Teff function and pancreatic residency, and long-term tolerance. TCR-driven gene expression controlling autoreactive Teff expansion and proinflammatory activity was suppressed by CoRT, and islet T cell egress was dependent on sphingosine-1 phosphate. In both murine and human T cells, CoRT upregulated the Foxo1 transcriptional axis, which in turn was required for suppression and efficient pancreatic egress of Teffs. Interestingly, long-term tolerance induced in late-preclinical NOD mice was marked by reseeding of the pancreas by a reduced CD8+ Teff pool exhibiting an exhausted phenotype. Notably, PD-1 blockade, which rescues exhausted Teffs, resulted in diabetes onset in protected animals. These findings demonstrate that CoRT has distinct intrinsic effects on Teffs that impact events early in induction and later in maintenance of self-tolerance.

Immune Checkpoint-Bioengineered Beta Cell Vaccine Reverses Early-Onset Type 1 Diabetes.

Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease that results from autoreactive T cells destroying insulin-producing pancreatic beta (ß) cells. The development of T1DM is associated with the deficiency of co-inhibitory immune checkpoint ligands (e.g., PD-L1, CD86, and Gal-9) in ß cells. Here, a new translational approach based on metabolic glycoengineering and bioorthogonal click chemistry, which bioengineers ß cells with co-inhibitory immune checkpoint molecules that induce antigen-specific immunotolerance and reverse early-onset hyperglycemia is reported. To achieve this goal, a subcutaneous injectable acellular pancreatic extracellular matrix platform for localizing the bioengineered ß cells while creating a pancreas-like immunogenic microenvironment, in which the autoreactive T cells can interface with the ß cells, is devised.

The Role of T Cell Receptor Signaling in the Development of Type 1 Diabetes.

T cell receptor (TCR) signaling influences multiple aspects of CD4+ and CD8+ T cell immunobiology including thymic development, peripheral homeostasis, effector subset differentiation/function, and memory formation. Additional T cell signaling cues triggered by co-stimulatory molecules and cytokines also affect TCR signaling duration, as well as accessory pathways that further shape a T cell response. Type 1 diabetes (T1D) is a T cell-driven autoimmune disease targeting the insulin producing ß cells in the pancreas. Evidence indicates that dysregulated TCR signaling events in T1D impact the efficacy of central and peripheral tolerance-inducing mechanisms. In this review, we will discuss how the strength and nature of TCR signaling events influence the development of self-reactive T cells and drive the progression of T1D through effects on T cell gene expression, lineage commitment, and maintenance of pathogenic anti-self T cell effector function.

Evolving Antibody Therapies for the Treatment of Type 1 Diabetes.

Type 1 diabetes (T1D) is widely considered to be a T cell driven autoimmune disease resulting in reduced insulin production due to dysfunction/destruction of pancreatic ß cells. Currently, there continues to be a need for immunotherapies that selectively reestablish persistent ß cell-specific self-tolerance for the prevention and remission of T1D in the clinic. The utilization of monoclonal antibodies (mAb) is one strategy to target specific immune cell populations inducing autoimmune-driven pathology. Several mAb have proven to be clinically safe and exhibit varying degrees of efficacy in modulating autoimmunity, including T1D. Traditionally, mAb therapies have been used to deplete a targeted cell population regardless of antigenic specificity. However, this treatment strategy can prove detrimental resulting in the loss of acquired protective immunity. Nondepleting mAb have also been applied to modulate the function of immune effector cells. Recent studies have begun to define novel mechanisms associated with mAb-based immunotherapy that alter the function of targeted effector cell pools. These results suggest short course mAb therapies may have persistent effects for regaining and maintaining self-tolerance. Furthermore, the flexibility to manipulate mAb properties permits the development of novel strategies to target multiple antigens and/or deliver therapeutic drugs by a single mAb molecule. Here, we discuss current and potential future therapeutic mAb treatment strategies for T1D, and T cell-mediated autoimmunity.

Lack of acute xenogeneic graft- versus-host disease, but retention of T-cell function following engraftment of human peripheral blood mononuclear cells in NSG mice deficient in MHC class I and II expression.

Immunodeficient mice engrafted with human peripheral blood mononuclear cells (PBMCs) support preclinical studies of human pathogens, allograft rejection, and human T-cell function. However, a major limitation of PBMC engraftment is development of acute xenogeneic graft- versus-host disease (GVHD) due to human T-cell recognition of murine major histocompatibility complex (MHC). To address this, we created 2 NOD- scid IL-2 receptor subunit γ ( IL2rg) null (NSG) strains that lack murine MHC class I and II [NSG-ß-2-microglobulin ( B2M) null ( IA IE)null and NSG -( Kb Db) null ( IAnull)]. We observed rapid human IgG clearance in NSG- B2Mnull ( IA IE) null mice whereas clearance in NSG -( Kb Db) null ( IAnull) mice and NSG mice was comparable. Injection of human PBMCs into both strains enabled long-term engraftment of human CD4+ and CD8+ T cells without acute GVHD. Engrafted human T-cell function was documented by rejection of human islet allografts. Administration of human IL-2 to NSG -( Kb Db) null ( IAnull) mice via adeno-associated virus vector increased human CD45+ cell engraftment, including an increase in human regulatory T cells. However, high IL-2 levels also induced the development of GVHD. These data document that NSG mice deficient in murine MHC support studies of human immunity in the absence of acute GVHD and enable evaluation of human antibody therapeutics targeting human T cells.-Brehm, M. A., Kenney, L. L., Wiles, M. V., Low, B. E., Tisch, R. M., Burzenski, L., Mueller, C., Greiner, D. L., Shultz, L. D. Lack of acute xenogeneic graft- versus-host disease, but retention of T-cell function following engraftment of human peripheral blood mononuclear cells in NSG mice deficient in MHC class I and II expression.

Therapies to Suppress ß Cell Autoimmunity in Type 1 Diabetes.

Type 1 diabetes (T1D) is an autoimmune disease that is generally considered to be T cell-driven. Accordingly, most strategies of immunotherapy for T1D prevention and treatment in the clinic have targeted the T cell compartment. To date, however, immunotherapy has had only limited clinical success. Although certain immunotherapies have promoted a protective effect, efficacy is often short-term and acquired immunity may be impacted. This has led to the consideration of combining different approaches with the goal of achieving a synergistic therapeutic response. In this review, we will discuss the status of various T1D therapeutic strategies tested in the clinic, as well as possible combinatorial approaches to restore ß cell tolerance.

Prevention of Type 1 Diabetes with Acetalated Dextran Microparticles Containing Rapamycin and Pancreatic Peptide P31.

Type 1 diabetes (T1D) is a common autoimmune disease with no cure. T1D subjects are dependent on daily exogenous insulin administration, due to the loss of functional insulin-producing ß cells. Needed are immunotherapies that prevent and/or treat T1D. One approach of immunotherapy is to administer an autoantigen to selectively tolerize diabetogenic effector T cells without global immunosuppression. To date, however, strategies of antigen-specific immunotherapy are largely ineffective in the clinic. Using an antigen-specific approach, a biodegradable polymeric delivery vehicle, acetalated dextran microparticles (Ace-DEX MPs), is applied and T1D development is prevented through coadministration of the immunosuppressant rapamycin and the diabetogenic peptide P31 (Rapa/P31/MPs), via alterations of both innate and adaptive immunity. Ex vivo, adoptively transferred CD4+ T cells exhibit reduced proliferation and an increased ratio of FoxP3+ to IFNγ+ T cells. In vitro analysis indicates dendritic cells exhibit a less mature phenotype following coculture with Rapa/P31/MPs, which results in reduced CD4+ T cell proliferation and proinflammatory cytokine production (IFNγ and IL-2), but promotes PD-1 expression. Together these results demonstrate Ace-DEX MP-based antigen-specific therapy effectively tolerizes diabetogenic CD4+ T cells to prevent T1D, thereby demonstrating one of the first successful attempts of T1D prevention using a single-formulation particulate delivery platform.

Gene Therapy with an Adeno-Associated Viral Vector Expressing Human Interleukin-2 Alters Immune System Homeostasis in Humanized Mice.

Recombinant adeno-associated viruses (rAAVs) serve as vectors for in vivo gene delivery in both mice and humans, and have broad applicability for the treatment of genetic diseases. Clinical trials with AAV vectors have demonstrated promise and safety in several human diseases. However, the in vivo validation of novel AAV constructs expressing products that act specifically on human cells and tissues is limited by a paucity of effective translatable models. Humanized mice that are engrafted with human cells, tissues, and immune systems offer strong potential to test the biological effectiveness of AAV vectors on human cells and tissues. Using the BLT (bone marrow, liver, thymus) humanized NOD-scid Il2rgnull (NSG) mouse model, which enables efficient development of HLA-restricted effector and regulatory T cells (Tregs), we have evaluated the delivery and function of human interleukin (IL)-2 by an AAV vector. Humanized mice treated with an AAV vector expressing human IL-2 showed a significant and sustained increase in the number of functional human FOXP3+CD4+ Tregs. The expression of human IL-2 did not significantly change the levels or activation status of conventional T-cell subsets. Numbers of activated human natural killer cells were also increased significantly in humanized mice treated with the IL-2 vector. These data recapitulate observations in clinical trials of IL-2 therapy and collectively show that humanized mouse models offer a translational platform for testing the efficacy of AAV vectors targeting human immune cells.

Type 1 Diabetes: A Chronic Anti-Self-Inflammatory Response.

Inflammation is typically induced in response to a microbial infection. The release of proinflammatory cytokines enhances the stimulatory capacity of antigen-presenting cells, as well as recruits adaptive and innate immune effectors to the site of infection. Once the microbe is cleared, inflammation is resolved by various mechanisms to avoid unnecessary tissue damage. Autoimmunity arises when aberrant immune responses target self-tissues causing inflammation. In type 1 diabetes (T1D), T cells attack the insulin producing ß cells in the pancreatic islets. Genetic and environmental factors increase T1D risk by in part altering central and peripheral tolerance inducing events. This results in the development and expansion of ß cell-specific effector T cells (Teff) which mediate islet inflammation. Unlike protective immunity where inflammation is terminated, autoimmunity is sustained by chronic inflammation. In this review, we will highlight the key events which initiate and sustain T cell-driven pancreatic islet inflammation in nonobese diabetic mice and in human T1D. Specifically, we will discuss: (i) dysregulation of thymic selection events, (ii) the role of intrinsic and extrinsic factors that enhance the expansion and pathogenicity of Teff, (iii) defects which impair homeostasis and suppressor activity of FoxP3-expressing regulatory T cells, and (iv) properties of ß cells which contribute to islet inflammation.

Thymic Dendritic Cell Subsets Display Distinct Efficiencies and Mechanisms of Intercellular MHC Transfer.

Thymic dendritic cells (DC) delete self-antigen-specific thymocytes, and drive development of Foxp3-expressing immunoregulatory T cells. Unlike medullary thymic epithelial cells, which express and present peripheral self-antigen, DC must acquire self-antigen to mediate thymic negative selection. One such mechanism entails the transfer of surface MHC-self peptide complexes from medullary thymic epithelial cells to thymic DC. Despite the importance of thymic DC cross-dressing in negative selection, the factors that regulate the process and the capacity of different thymic DC subsets to acquire MHC and stimulate thymocytes are poorly understood. In this study intercellular MHC transfer by thymic DC subsets was investigated using an MHC-mismatch-based in vitro system. Thymic conventional DC (cDC) subsets signal regulatory protein α (SIRPα+) and CD8α+ readily acquired MHC class I and II from thymic epithelial cells but plasmacytoid DC were less efficient. Intercellular MHC transfer was donor-cell specific; thymic DC readily acquired MHC from TEC plus thymic or splenic DC, whereas thymic or splenic B cells were poor donors. Furthermore DC origin influenced cross-dressing; thymic versus splenic DC exhibited an increased capacity to capture TEC-derived MHC, which correlated with direct expression of EpCAM by DC. Despite similar capacities to acquire MHC-peptide complexes, thymic CD8α+ cDC elicited increased T cell stimulation relative to SIRPα+ cDC. DC cross-dressing was cell-contact dependent and unaffected by lipid raft disruption of donor TEC. Furthermore, blocking PI3K signaling reduced MHC acquisition by thymic CD8α+ cDC and plasmacytoid DC but not SIRPα+ cDC. These findings demonstrate that multiple parameters influence the efficiency of and distinct mechanisms drive intercellular MHC transfer by thymic DC subsets.

ß-cell-specific IL-35 therapy suppresses ongoing autoimmune diabetes in NOD mice.

IL-35 is a recently identified cytokine exhibiting potent immunosuppressive properties. The therapeutic potential and effects of IL-35 on pathogenic T effector cells (Teff) and Foxp3+ Treg, however, are ill defined. We tested the capacity of IL-35 to suppress ongoing autoimmunity in NOD mice. For this purpose, an adeno-associated virus vector in which IL-35 transgene expression is selectively targeted to ß cells via an insulin promoter (AAV8mIP-IL35) was used. AAV8mIP-IL35 vaccination of NOD mice at a late preclinical stage of type 1 diabetes (T1D) suppressed ß-cell autoimmunity and prevented diabetes onset. Numbers of islet-resident conventional CD4+ and CD8+ T cells, and DCs were reduced within 4 weeks of AAV8mIP-IL35 treatment. The diminished islet T-cell pool correlated with suppressed proliferation, and a decreased frequency of IFN-γ-expressing Teff. Ectopic IL-35 also reduced islet Foxp3+ Treg numbers and proliferation, and protection was independent of induction/expansion of adaptive islet immunoregulatory T cells. These findings demonstrate that IL-35-mediated suppression is sufficiently robust to block established ß-cell autoimmunity, and support the use of IL-35 to treat T1D and other T-cell-mediated autoimmune diseases.

Staphylococcus aureus Protein A Disrupts Immunity Mediated by Long-Lived Plasma Cells.

Infection with Staphylococcus aureus does not induce long-lived protective immunity for reasons that are not completely understood. Human and murine vaccine studies support a role for Abs in protecting against recurring infections, but S. aureus modulates the B cell response through expression of staphylococcus protein A (SpA), a surface protein that drives polyclonal B cell expansion and induces cell death in the absence of costimulation. In this murine study, we show that SpA altered the fate of plasmablasts and plasma cells (PCs) by enhancing the short-lived extrafollicular response and reducing the pool of bone marrow (BM)-resident long-lived PCs. The absence of long-lived PCs was associated with a rapid decline in Ag-specific class-switched Ab. In contrast, when previously inoculated mice were challenged with an isogenic SpA-deficient S. aureus mutant, cells proliferated in the BM survival niches and sustained long-term Ab titers. The effects of SpA on PC fate were limited to the secondary response, because Ab levels and the formation of B cell memory occurred normally during the primary response in mice inoculated with wild-type or SpA-deficient S. aureus mutant. Thus, failure to establish long-term protective Ab titers against S. aureus was not a consequence of diminished formation of B cell memory; instead, SpA reduced the proliferative capacity of PCs that entered the BM, diminishing the number of cells in the long-lived pool.

Antibody Binding to CD4 Induces Rac GTPase Activation and Alters T Cell Migration.

The use of nondepleting Abs specific for CD4 and CD8 is an effective strategy to tolerize CD4+ and CD8+ T cells in a tissue-specific manner. We reported that coreceptor therapy reverses diabetes in new onset NOD mice. A striking feature of coreceptor-induced remission is the purging of T cells from the pancreatic lymph nodes (PLN) and islets of NOD mice. Evidence indicates that Abs binding to the coreceptors promotes T cell egress from these tissues. The present study examined how coreceptor therapy affects the migration of CD4+ T cells residing in the PLN of NOD mice. Anti-CD4 Ab treatment resulted in an increased frequency of PLN but not splenic CD4+ T cells that exhibited a polarized morphology consistent with a migratory phenotype. Furthermore, PLN CD4+ T cells isolated from anti-CD4 versus control Ab-treated animals displayed increased in vitro chemotaxis to chemoattractants such as sphingosine-1-phosphate and CXCL12. Notably, the latter was dependent on activation of the small Rho GTPases Rac1 and Rac2. Rac1 and Rac2 activation was increased in Ab-bound CD4+ T cells from the PLN but not the spleen, and knockdown of Rac expression blocked the heightened reactivity of Ab-bound PLN CD4+ T cells to CXCL12. Interestingly, Rac1 and Rac2 activation was independent of Rac guanine nucleotide exchange factors known to regulate T cell activity. Therefore, Ab binding to CD4 initiates a novel pathway that involves inflammation-dependent activation of Rac and establishment of altered T cell migratory properties.