HIV-1 Vpr enhances viral burden by facilitating infection of tissue macrophages but not nondividing CD4+ T cells.

Prior experiments in explants of human lymphoid tissue have demonstrated that human immunodeficiency virus type 1 (HIV-1) productively infects diverse cellular targets including T cells and tissue macrophages. We sought to determine the specific contribution of macrophages and T cells to the overall viral burden within lymphoid tissue. To block infection of macrophages selectively while preserving infection of T cells, we used viruses deficient for viral protein R (Vpr) that exhibit profound replication defects in nondividing cells in vitro. We inoculated tonsil histocultures with matched pairs of congenic viruses that differed only by the presence of a wild-type or truncated vpr gene. Although these viruses exhibited no reduction in the infection or depletion of T cells, the ability of the Vpr-deficient R5 virus to infect tissue macrophages was severely impaired compared with matched wild-type R5 virus. Interestingly, the Vpr-deficient R5 virus also exhibited a 50% reduction in overall virus replication compared with its wild-type counterpart despite the fact that macrophages represent a small fraction of the potential targets of HIV-1 infection in these tissues. Collectively, these data highlight the importance of tissue macrophages in local viral burden and further implicate roles for CC chemokine receptor 5, macrophages, and Vpr in the life cycle and pathogenesis of HIV-1.

Early signal transduction by the antigen receptor without commitment to T cell activation.

The T lymphocyte antigen-receptor complex mediates antigen-specific cell activation, at least in part, through the production of inositolphospholipid-derived second messengers. Little is known about how second messenger events, typically measured within minutes of ligand binding, eventually lead to distal biologic responses such as expression of lymphokine genes. Several monoclonal antibodies directed against the receptor complex were tested for their ability to elicit transmembrane signaling in the parental Jurkat line and in a somatic mutant (J.CaM1) with a deficient receptor function. One antibody elicited substantial early Ca2+ mobilization responses in both cells but was unable to promote expression of the interleukin-2 gene in J.CaM1. In J.CaM1 there was a diminished production of phosphatidylinositol second messengers, and the elevation in intracellular free Ca2+ was transient. Thus, short-term Ca2+ mobilization does not always indicate complete signal transmission and lead to a full cellular response.

Multiple extracellular elements of CCR5 and HIV-1 entry: dissociation from response to chemokines.

The human beta-chemokine receptor CCR5 is an important cofactor for entry of human immunodeficiency virus-type 1 (HIV-1). The murine form of CCR5, despite its 82 percent identity to the human form, was not functional as an HIV-1 coreceptor. HIV-1 entry function could be reconstituted by fusion of various individual elements derived from the extracellular region of human CCR5 onto murine CCR5. Analysis of chimeras containing elements from human CCR5 and human CCR2B suggested that a complex structure rather than single contact sites is responsible for facilitation of viral entry. Further, certain chimeras lacking the domains necessary to signal in response to their natural chemokine ligands retained vigorous HIV-1 coreceptor activity.

Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation.

BACKGROUND: Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. Furthermore, the relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) remain poorly understood. RESULTS: We targeted two conserved tyrosine residues within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly required the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retained catalytic activity when either or both of these activation-loop tyrosines were mutated. Analysis of JAK1/3 chimeras demonstrated that JAK activity was also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. Finally, we have reconstituted IL-2-dependent STAT5 induction in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5. CONCLUSIONS: JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation.

Parallel evolution of CCR5-null phenotypes in humans and in a natural host of simian immunodeficiency virus.

The C-C chemokine receptor CCR5 in humans and rhesus macaques (Macaca mulatta) serves as the primary coreceptor for cellular entry by macrophagetropic strains of human immunodeficiency virus type 1 (HIV-1) and all reported strains of simian immunodeficiency virus (SIV) [1-6]. Humans homozygous for a 32 bp deletion allele of CCR5, resulting in a null phenotype, are highly resistant to infection by HIV-1 [7-9], prompting development of therapies and vaccines targeting CCR5. We now report a novel deletion allele of CCR5, with an allele frequency of 0.04, in sooty mangabey monkeys (Cercocebus torquatus atys), a natural host of SIV (SIVsmm) [10]. The mutant protein was not expressed at the cell surface and accordingly did not function as a viral coreceptor. Primary activated lymphocytes from mangabeys heterozygous for the deletion allele expressed significantly less CCR5 on the cell surface. Moreover, SIV seroprevalence and viremia were comparable among CCR5 heterozygotes and wild-type animals. Parallel evolution of CCR5-null alleles in humans and sooty mangabeys suggests that similar negative selection pressures have acted against CCR5, as would occur during epidemics of infectious agents that require CCR5 for pathogenesis. Sooty mangabeys bred to homozygosity for the deletion allele will be useful for experimental studies on the context-dependent role of CCR5 in host defense and microbial pathogenesis.

A trans-receptor mechanism for infection of CD4-negative cells by human immunodeficiency virus type 1.

Chemokine receptors, particularly CCR5 and CXCR4, act as essential coreceptors in concert with CD4 for cellular entry by human immunodeficiency virus type 1 (HIV-1; reviewed in [1]). But infection of CD4(-) cells has also been encountered in various tissues in vivo, including astrocytes, neurons and microvascular endothelial cells of the brain [2] [3] [4] [5] [6], epithelial cells [5] [7], CD4(-) lymphocytes and thymocytes [8] [9], and cardiomyocytes [10]. Here, we present evidence for the infection of CD4(-) cell lines bearing coreceptors by well-known HIV-1 strains when co-cultured with CD4(+) cells. This process requires contact between the coreceptor-bearing and CD4(+) cells and supports the full viral replication cycle within the coreceptor-bearing target cell. Furthermore, CD4 provided in trans facilitates infection of primary human cells, such as brain-derived astrocytes. Although the pathobiological significance of infection of CD4(-) cells in vivo remains to be elucidated, this trans-receptor mechanism may facilitate generation of hidden reservoirs of latent virus that confound antiviral therapies and that contribute to specific AIDS-associated clinical syndromes.

Shared gamma(c) subunit within the human interleukin-7 receptor complex. A molecular basis for the pathogenesis of X-linked severe combined immunodeficiency.

Genetic evidence suggests that mutations in the gamma(c) receptor subunit cause X-linked severe combined immunodeficiency (X-SCID). The gamma(c) subunit can be employed in receptor complexes for IL-2, -4, -7, -9, and -15, and the multiple signaling defects that would result from a defective gamma(c) chain in these receptors are proposed to cause the severe phenotype of X-SCID patients. Interestingly, gene disruption of either IL-7 or the IL-7 receptor (IL-7R) alpha subunit in mice leads to immunological defects that are similar to human X-SCID. These observations suggest the functional importance of gamma(c) in the IL-7R complex. In the present study, structure/function analyses of the IL-7R complex using a chimeric receptor system demonstrated that gamma(c) is indeed critical for IL-7R function. Nonetheless, only a limited portion of the cytoplasmic domain of gamma(c) is necessary for IL-7R signal transduction. Furthermore, replacement of the gamma(c) cytoplasmic domain by a severely truncated erythropoeitin receptor does not affect measured IL-7R signaling events. These findings support a model in which gamma(c) serves primarily to activate signal transduction by the IL-7R complex, while IL-7R alpha determines specific signaling events through its association with cytoplasmic signaling molecules. Finally, these studies are consistent with the hypothesis that the molecular pathogenesis of X-SCID is due primarily to gamma(c)-mediated defects in the IL-7/IL-7R system.

Abnormal polyamine metabolism in hereditary muscular dystrophies: effect of human growth hormone.

Previous studies showed hyperre-sponsiveness to human growth hormone (hGH) in men with myotonic or limb girdle dystrophies (MMD or LGD). Because polyamines may mediate some actions of hGH, we have now investigated polyamine metabolism in these and other dystrophies. Under metabolic balance study conditions, serum and urine levels of putrescine (Pu), spermidine (Sd), spermine (Sm), and cadaverine (Cd) were measured in six normal men (36-44 yr), four men with MMD (38-44 yr), and three men with LGD (30-36 yr), before and during treatment with 0.532 U/kg body wt ((3/4)/d) of hGH. Daily balances of N, P, and K were also monitored. In the normal subjects, hGH did not influence elemental balances or serum and urine polyamines. In MMD, hGH caused significant retention of N, P, and K (P < 0.005). Basal levels of Sm and Cd were significantly elevated above normal (P < 0.005), and Pu, Sm, and Cd increased two- to fourfold above basal during hGH treatment (P < 0.005). In LGD, hGH also caused retention of N, P, and K. Basal levels of nearly all the polyamines (not serum Pu) were significantly above normal in serum and urine (P < 0.05). During hGH treatment, all four polyamines rose significantly above basal (P < 0.005). Serum and urine polyamine levels in five boys with Duchenne muscular dystrophy, age 8-13, did not differ from those in five age-matched normal boys. Skeletal muscle polyamines were measured in five men (31-40 yr) without muscle disease and in three men with LGD (30-38 yr). Average concentrations of Pu, Sd, Sm, and Cd were 46, 306, 548, and 61 nmol/g wet wt in LGD and 1, 121, 245, and 14 in the normal subjects, respectively (P < 0.05 in each instance). Polyamines were determined in skeletal muscle, liver, kidney, and brain of male mice with hereditary muscular dystrophy and in age- and sex-matched normal controls. Pu, Sd, Sm, and Cd levels were two to three times higher than normal in muscle, but did not differ in liver, kidney, and brain. Similar findings were made in male hamsters with hereditary dystrophy and in their controls. The abnormality in hamster muscle polyamines appeared between 1 and 6 wk of age and persisted or intensified until 30 wk. These data reveal abnormalities of polyamine metabolism in men with MMD, in men with LGD, and in mice or hamsters with hereditary muscular dystrophy. The polyamine disorder could be related to dystrophic patients' hyperresponsiveness to hGH.

Serum and urine polyamines in normal and in short children.

The serum and urine polyamines putrescine, spermidine, and spermine were measured in 112 normal subjects from 0 to 70 yr of age, and in three groups of short children from 7 to 20 yr: 21 growth hormone (GH) deficient patients, 20 normal variant short stature children, and 9 girls with 45, X Turner's syndrome. Urine polyamines were expressed as micromoles per gram of creatinine or per kilogram body weight, and serum polyamines were expressed as nanomoles per milliliter. In normals, the three polyamines were highest in urine and serum at birth. The mean levels declined progressively with age, the rate of change decreasing with age. The mean for the normal subjects, and its 95% confidence and prediction intervals, were estimated from birth to age 70 for each serum and urine polyamine. In GH-deficient children, serum and urine values were significantly lower (P < 0.05) than the age-specific normal values (with the exception of serum spermidine and spermine), averaging 25-55% below normal. This abnormality was corrected during 1 wk of treatment with human GH. In Turner's syndrome, serum and urine values were significantly reduced (P < 0.05), averaging 35-80% below age-specific normals. GH treatment had no corrective effect. In 6 of 20 normal variant short stature children, polyamine levels were significantly (P < 0.01) subnormal, averaging 50-80% below age-specific normals in both serum and urine. Treatment with GH had no corrective effect. These data show that levels of polyamines in serum and urine are correlated with linear growth primarily during the first decade of life. Subnormal polyamine levels are generally associated with growth retardation.

JAK/STAT signaling by cytokine receptors.

The JAK/STAT pathway is recognized as one of the major mechanisms by which cytokine receptors transduce intracellular signals. This system is regulated at multiple levels, including JAK activation, nuclear trafficking of STAT factors, and negative feedback loops. Gene deletion studies have implicated selected STAT factors as predominant mediators for a limited number of lymphokines. This signaling pathway influences normal cell survival and growth mechanisms and may contribute to oncogenic transformation.

Quantitative prediction of drug toxicity in humans from toxicology in small and large animals.

The mouse, dog, and monkey toxicity data on 30 drugs was retrospectively analyzed in comparison with the actual clinical dose schedules used in man. Animal dose schedules were converted to the human schedule and comparisons were made of the human dose versus the large animal toxic dose low, toxic dose high, and lethal dose, the lethal doses for 10% and 90% of normal mice, and the optimal dose in tumor-bearing mice. If the starting dose in Phase 1 clinical trials had been selected by calculating one-third of the toxic dose low (in mg/sq m) in the most sensitive large animal species, 5 of the 30 drugs would have produced significant toxicity in the first patient. The lethal doses for 10 and 90% of normal mice and the optimal dose in L1210-bearing mice were found to offer good quantitative prediction of human toxicity. Determination of a safe and practical starting dose for Phase 1 studies should take into account not only dog and monkey data but also toxicology data in normal and tumor-bearing mice.

Genomic cloning, structure, and regulatory elements of the 1 alpha, 25(OH)2D3 down-regulated gene for cyclic AMP-dependent protein kinase inhibitor.

The cyclic AMP-dependent protein kinase inhibitor (PKI) mRNA and protein are negatively and tissue-specifically regulated in the kidney by 1 alpha, 25(OH)2D3. A 17-kb PKI clone, isolated from a chick genomic library, revealed that the PKI gene consists of two exons separated by a 4.5-kb intron. A 411-bp upstream region (constituting 93 bp upstream and 318 bp downstream from the transcriptional start site) containing a putative negative VDRE (nVDRE) fused to the luciferase gene was used for transient transfections of primary cultures of chick kidney cells. Luciferase activity was significantly down-regulated in response to 1 alpha, 25(OH)2D3. This result suggests that the promoter region containing the putative nVDRE plays a pivotal role in the negative regulation of PKI gene transcription.

Soluble type II transforming growth factor-beta (TGF-beta) receptor inhibits TGF-beta signaling in COLO-357 pancreatic cancer cells in vitro and attenuates tumor formation.

UNLABELLED: Human pancreatic ductal adenocarcinomas overexpress transforming growth factor-betas (TGF-betas). This overexpression has been correlated with decreased patient survival. TGF-betas bind to a type II TGF-beta receptor (TbetaRII) dimer, which heterotetramerizes with a type I TGF-beta receptor (TbetaRI) dimer, thereby activating downstream signaling. PURPOSE AND EXPERIMENTAL DESIGN: To determine whether blocking TGF-beta actions would suppress pancreatic cancer cell growth in vivo, we expressed a soluble TbetaRII, encoding amino acids 1-159 of the extracellular domain in COLO-357 human pancreatic cancer cells. This cell line expresses all of the three mammalian TGF-beta isoforms and is growth inhibited by TGF-beta in vitro. RESULTS: COLO-357 clones expressing soluble TbetaRII were no longer growth inhibited by exogenous TGF-beta1 and exhibited a marked decrease in their invasive capacity in vitro. When injected s.c. into athymic mice, these clones exhibited attenuated growth rates and angiogenesis and decreased levels of plasminogen activator inhibitor-1 mRNA as compared with tumors formed by sham-transfected cells. CONCLUSIONS: These results indicate that endogenous TGF-betas can confer a growth advantage in vivo to a pancreatic cancer cell line that is growth inhibited in vitro and suggest that a soluble receptor approach can be used to block these tumorigenic effects of TGF-betas.

The effects of proton pump inhibition on patient-reported severity of dyspepsia when receiving dual anti-platelet therapy with clopidogrel and low-dose aspirin: analysis from the Clopidogrel and the Optimization of Gastrointestinal Events Trial.

BACKGROUND: Dual anti-platelet therapy with clopidogrel and low-dose aspirin increases the risk for gastrointestinal clinical events. Omeprazole has been shown to significantly reduce these events without compromising cardiovascular safety in patients treated with dual anti-platelet therapy. Whether or not omeprazole improves patient-reported outcomes is undetermined. AIM: To assess the impact of prophylactic omeprazole with background dual anti-platelet therapy on patient-reported symptoms of dyspepsia compared to placebo. METHODS: We analysed results of the Severity of Dyspepsia Assessment questionnaires collected in the Clopidogrel and the Optimization of Gastrointestinal Events Trial. RESULTS: Patient-reported outcome data from 3759 subjects were available for analysis. At 4 weeks, the mean scores of pain intensity and nonpain symptoms were lower in the omeprazole group (5.61 ± 0.17 vs. 6.40 ± 0.17, P = 0.001, and 10.61 ± 0.07 vs. 11.00 ± 0.07, P < 0.001 respectively). These differences were maintained at 24 weeks (5.91 ± 0.35 vs. 7.10 ± 0.37, P = 0.020 for pain intensity; 10.36 ± 0.12 vs. 10.93 ± 0.13, P = 0.001 for nonpain symptoms). After adjusting for covariates there were no statistically significant differences between the groups in the percent of patients with dyspepsia during follow-up. CONCLUSIONS: In addition to reducing the risk of gastrointestinal bleeding, statistically significant benefits with prophylactic omeprazole use on both pain and nonpain symptoms were evident at 4 weeks and sustained through 24 weeks. The clinical significance of these overall results is unclear, but greater in patients with pain at baseline.

Predicting the response of growth hormone-deficient children to long term treatment with human growth hormone.

A previous study showed that when GH-deficient children below the third percentile in height are treated with 0.168 U human GH (hGH)/kg BW3/4 for 10 days, their height increases by 0.3--1.9 cm during the next 8 weeks. The present study determined whether this acute response would predict the child's long term response to 1 yr of treatment with the same dose of hGH given three times a week. Eighteen GH-deficient children and adolescents, aged 8--16 yr, were measured every 2 weeks over 108 weeks. After a control period of 12 weeks (period 1), the patient received hGH for 10 days. During the remainder of the 12 weeks of period 2 and during the next 12 weeks (period 3), hGH was not given. Patients recieved hGH three times a week during periods 4 and 5 (24 weeks each). Periods 6 and 7 (12 weeks each) were posttreatment control periods. During periods 1, 3, 6, and 7, rate of growth was less than 0.2 cm/month. During period 2, the rate ranged between 0.1--0.8 cm/month. During periods 4 and 5, the growth rate ranged from 0.2--1.0 cm/month. Rate of growth during periods 4 and 5 (y) was related to rate during period 2 (x) by the equation y = 0.027 + 1.17 x. The correlation coefficient between y and x was 0.91 (P less than 0.001). The increment in height which will occur during 48 weeks of treatment can be predicted from the response to 10 days of treatment by this equation. The SE of the prediction averages +/- 1.2 cm/yr.

Molecular characterization of a Leydig cell tumor presenting as congenital adrenal hyperplasia.

We present an unusual patient with a Leydig cell tumor to show that greatly elevated serum concentrations of 17-hydroxyprogesterone (17OHP) may not be diagnostic of congenital adrenal hyperplasia (CAH). A 3.5-yr-old boy had a small testicular mass and plasma 17OHP concentrations of 147-333 nmol/L (4,850-11,000 ng/dL), suggesting CAH with adrenal rests. However, normal plasma cortisol values and the unresponsiveness of the 17OHP concentration to dexamethasone suppression or ACTH stimulation suggested a diagnosis of Leydig cell tumor. A 4-fold elevation in plasma 21-deoxycortisol compared with a 200-fold elevation in 17OHP suggested that the elevated 17OHP derived from the normal pathway of testosterone synthesis in the testis. This was proven by normalization of all hormonal values after tumor resection. Compared to the abundance of mRNA for P450c17, the tumor contained unusually large amounts of mRNA for P450scc, the cholesterol side-chain cleavage enzyme, which is the rate-limiting step in steroid hormone synthesis. Increased P450scc activity, which increased the conversion of cholesterol to pregnenolone, apparently permitted the 17,20-lyase activity of P450c17 to become rate limiting, thus accounting for the increased secretion of 17OHP. Thus, Leydig cell tumors can produce quantities of 17OHP previously reported only in CAH due to 21-hydroxylase deficiency. The molecular characterization of steroidogenic mRNAs in this tumor indicates an unusual ratio in the expression of the genes for the steroidogenic enzymes, probably accounting for the unusual pattern of serum steroids.

A method of screening for growth hormone deficiency using anthropometrics.

Among children less than 3rd percentile in height, less than 1% are deficient in endogenous growth hormone, while 80% have no recognizable organic cause for short stature, and are termed normal variants. This study investigated whether anthropometric evaluation can distinguish growth hormone-deficient from normal variant children. Height, weight, midarm circumference and 10 skinfold thicknesses were measured in 24 growth hormone-deficient and 26 normal variant children; indices of linear growth, adiposity, and lean body mass were derived from these. All these variables were analyzed statistically by discriminant analysis. This led to a screening formula, here called a "Z-function," based only on height and five skinfolds (abdomen, back, chest, knee, and calf). The Z-function classified correctly all but two growth hormone-deficients and two normal variants. Because of the small and possibly inhomogeneous sample, the particular formula developed here is not recommended for general use, but these preliminary findings show that a simple anthropometric screening test is indeed feasible, and could be useful in pediatric practice.

Molecular function of the CD4 D1 domain in coreceptor-mediated entry by HIV type 1.

The surface molecule CD4 plays a key role in initiating cellular entry by the human immunodeficiency virus type 1 (HIV-1), and it is now recognized as acting synergistically with select chemokine receptors (coreceptors) in the infection process. The present study was undertaken to determine whether the extracellular region of CD4 is sufficient to induce fusion of HIV-1 virions with target cells in the absence of its anchoring function. Using pseudotype reporter viruses to quantitate infection, soluble CD4 (sCD4) was tested for its ability to induce fusion by viruses utilizing CCR5 as their coreceptor. We found that sCD4 was competent to replace membrane-bound CD4 to trigger infection mediated by several HIV-1 envelopes. Furthermore, in a comparison of the envelopes of HIV-1 NL4-3 and a chimera containing the gp120 V3 loop of Ba-L, the V3 region was found to be one factor affecting susceptibility to induction by sCD4. In addition, using truncated and mutant derivatives of sCD4, the amino-terminal D1 domain of CD4 was found to be necessary and sufficient for induction of fusion and to require an intact gp120-binding site for this activity. These results delineate determinants on CD4 and gp120 required for fusion induction in collaboration with a coreceptor, and suggest a mechanism whereby CD4 may contribute to viral infection in trans.

Primary and recombinant HIV type 1 strains resistant to protease inhibitors are pathogenic in mature human lymphoid tissues.

Preserved peripheral CD4+ T cell counts despite virologic failure in patients undergoing protease inhibitor (PI)-containing antiviral regimens are a frequent occurrence in human immunodeficiency virus (HIV) disease. One hypothesis to explain the relative sparing of CD4+ T cells is that HIV strains exhibiting PI resistance concomitantly are attenuated in terms of cytopathicity for mature T cells. To test this hypothesis, we used a three-dimensional human tonsil histoculture microenvironment to assess the pathogenic potential of a panel of primary and recombinant HIV-1 strains derived from patients experiencing PI failure. All the viruses tested replicated efficiently in these cultures and, in some cases, better than comparable wild-type viral isolates. Furthermore, the PI-resistant strains depleted CD4+ T cells potently and comparably with wild-type isolates in these ex vivo lymphoid tissues. These results demonstrate that PI-resistant viruses are not inherently less pathogenic for mature T cells. Therefore, the sustained peripheral lymphocyte counts in patients with selective virologic failure may be due to specific defects in viral replication in other cell compartments or to an undefined host adaptation to viral infection during PI therapy.

The activation of T lymphocytes.

The events involved in T cell activation are initiated at the cell surface by the interaction of ligands with specific cell surface receptors on the T cell. Central to antigen-induced activation is the CD3/Ti complex, a complex multi-chain receptor responsible for antigen/MHC recognition and signal transduction. Triggering the CD3/Ti complex results in the generation of intracellular second messengers, IP3 and DG, which are derived from PI metabolism. The second messengers lead to increases in [Ca2+]i and activation of pkC, events causally linked to various cellular responses, including the production of IL-2 through as yet poorly defined pathways. Little is known about how other cell surface molecules that may provide an accessory function participate in such events. However, future genetic and biochemical studies are likely to shed light upon the mechanisms of signal transduction by the CD3/Ti complex and accessory molecules and the details of the intracellular events involved in the activation of a host of cellular genes associated with activation.