The job search problem as an employer–candidate game

In the standard job search problem a single decision-maker (say an employer) has to choose from a sequence of candidates of varying fitness. We extend this formulation to allow both employers and candidates to make choices. We consider an infinite population of employers and an infinite population of candidates. Each employer interviews a (possibly infinite) sequence of candidates for a post and has the choice of whether or not to offer a candidate the post. Each candidate is interviewed by a (possibly infinite) sequence of employers and can accept or reject each offer. Each employer seeks to maximise the fitness of the candidate appointed and each candidate seeks to maximise the fitness of their eventual employer. We allow both discounting and/or a cost per interview. We find that there is a unique pair of policies (for employers and candidates respectively) which is in Nash equilibrium. Under these policies each population is partitioned into a finite or countable sequence of subpopulations, such that an employer (candidate) in a given subpopulation ends up matched with the first candidate (employer) encountered from the corresponding subpopulation. In some cases the number of non-empty subpopulations in the two populations will differ and some members of one population will never be matched.

An analysis of transient Markov decision processes

This paper is concerned with the analysis of Markov decision processes in which a natural form of termination ensures that the expected future costs are bounded, at least under some policies. Whereas most previous analyses have restricted attention to the case where the set of states is finite, this paper analyses the case where the set of states is not necessarily finite or even countable. It is shown that all the existence, uniqueness, and convergence results of the finite-state case hold when the set of states is a general Borel space, provided we make the additional assumption that the optimal value function is bounded below. We give a sufficient condition for the optimal value function to be bounded below which holds, in particular, if the set of states is countable.

The Hawk–Dove game as an average-cost problem

This paper considers a version of the Hawk–Dove game of Maynard Smith and Price (1973) in which animals compete for a sequence of food items. Actions may depend on an animal's energy reserves. Costs and transition probabilities under a given policy depend on the mean level of aggressiveness, p, of the rest of the population. We find the optimal policy for a single animal under an average cost criterion when ρ is constant over time. We then consider the whole interacting population when individual members follow the same stationary policy. It is shown that the mean aggressiveness, p, asymptotically approaches a limiting value in this population. We then consider the existence of evolutionarily stable strategies for the population. It is shown that such strategies always exist but may not be unique.

Finite-horizon dynamic optimisation when the terminal reward is a concave functional of the distribution of the final state

We consider a problem similar in many respects to a finite horizon Markov decision process, except that the reward to the individual is a strictly concave functional of the distribution of the state of the individual at final time T. Reward structures such as these are of interest to biologists studying the fitness of different strategies in a fluctuating environment. The problem fails to satisfy the usual optimality equation and cannot be solved directly by dynamic programming. We establish equations characterising the optimal final distribution and an optimal policy π*. We show that in general π* will be a Markov randomised policy (or equivalently a mixture of Markov deterministic policies) and we develop an iterative, policy improvement based algorithm which converges to π*. We also consider an infinite population version of the problem, and show that the population cannot do better using a coordinated policy than by each individual independently following the individual optimal policy π*.

The job-search problem with competition: an evolutionarily stable dynamic strategy

We consider a game-theoretical solution for an optimal stopping problem which we describe in terms of a job-search problem with an infinite population of candidates and an infinite population of posts of varying value. Each candidate finds posts from the post population at unit rate. If a post found is still vacant, a candidate can either accept it or reject it. The reward to a candidate is the value of the post if one is accepted and zero if he never accepts a post. There are no costs for searching, no discounting of future rewards and no recall of previously found posts. The only pressure on a candidate to accept a post comes from the changing rate at which he finds vacant posts (and the values associated with them) as a result of the actions of the other candidates. It is not possible to define optimality for a single candidate without reference to the policies followed by the other candidates. We say a policy π is an evolutionarily stable strategy (ESS) if it has the following property: if all candidates used π and an individual candidate was given the option of changing his policy, then it would not be to his advantage to do so. We first find the optimal value function and optimal policy for the case of a single candidate operating in an environment where the distribution of posts on offer and the chance of finding one both vary with time in a known way. We then show that for the infinite population there is a unique ESS given by a control-limit policy π c, where the control-limit function c is the solution of a given differential equation with a given initial condition. This function c also gives the expected future reward function for any single candidate when all candidates use π c.

Enhancing cytotoxic T cell responses with altered-peptide ligands

Interest in class I MHC-mediated immunotherapy is growing rapidly. In order to fight a virus or cancer effectively, a successful immunotherapeutic must activate a large number of specific CD8+ T cells and also generate immunological memory. Attempts to generate immune responses towards tumor- or virus-derived peptides have frequently been frustrated by the nature of the peptide antigen itself. Either the peptide does not bind well to its cognate MHC, or the T cells directed towards it have been functionally inactivated in vivo. Altered-peptide ligands (APL) are an effective way to circumvent these problems. However, generating enhanced binding of altered peptides to class I MHC while still maintaining recognition of the wild-type peptide is not straightforward. Many groups design enhanced binding peptides by substituting the observed anchor residues with those that are most preferred by the class I MHC molecule. For many antigenic peptides, this approach does not work. Furthermore, if a higher affinity peptide is designed, the substitutions may result in reduced recognition by CD8+ T cells. Therefore, the design of APL requires careful testing of each candidate therapeutic in terms of affinity for class I MHC and immunological reactivity. Lastly, immunotherapy using class I MHC must also take into account the large genetic heterogeneity in the population. A therapeutic that is only effective for 5-10% of the population is not as attractive as one that works for over 90% of the population. The use of MHC supertypes (groups of class I MHC allotypes that share similar peptide-binding characteristics) shows great promise in overcoming this problem.

T cell activity correlates with oligomeric peptide-major histocompatibility complex binding on T cell surface

Recognition of virally infected cells by CD8+ T cells requires differentiation between self and nonself peptide-class I major histocompatibility complexes (pMHC). Recognition of foreign pMHC by host T cells is a major factor in the rejection of transplanted organs from the same species (allotransplant) or different species (xenotransplant). AHIII12.2 is a murine T cell clone that recognizes the xenogeneic (human) class I MHC HLA-A2.1 molecule (A2) and the syngeneic murine class I MHC H-2 D(b) molecule (D(b)). Recognition of both A2 and D(b) are peptide-dependent, and the sequences of the peptides recognized have been determined. Alterations in the antigenic peptides bound to A2 cause large changes in AHIII12.2 T cell responsiveness. Crystal structures of three representative peptides (agonist, null, and antagonist) bound to A2 partially explain the changes in AHIII12.2 responsiveness. Using class I pMHC octamers, a strong correlation is seen between T cell activity and the affinity of pMHC complexes for the T cell receptor. However, contrary to previous studies, we see similar half-lives for the pMHC multimers bound to the AHIII12.2 cell surface.

Treatment-resistance to clozapine in association with ultrarapid CYP1A2 activity and the C-->A polymorphism in intron 1 of the CYP1A2 gene: effect of grapefruit juice and low-dose fluvoxamine

Antipsychotic response to clozapine varies markedly among patients with schizophrenia. The disposition of clozapine is dependent, in part, on the cytochrome P-450 (CYP) 1A2 enzyme in vivo. In theory, a very high CYP1A2 activity may lead to subtherapeutic concentrations and treatment resistance to clozapine. This prospective case study evaluates the clinical significance of ultrarapid CYP1A2 activity and a recently discovered single nucleotide (C --> A) polymorphism in intron 1 of the CYP1A2 gene (CYP1A2*F) for treatment resistance to clozapine. In addition, we describe the effect of grapefruit juice or low-dose fluvoxamine (25-50 mg/d) coadministration on clozapine and active metabolite norclozapine steady-state plasma concentration and antipsychotic response.

Antigen-specific modulation of an immune response by in vivo administration of soluble MHC class I tetramers

Soluble MHC/peptide tetramers that can directly bind the TCR allow the direct visualization and quantitation of Ag-specific T cells in vitro and in vivo. We used HY-D(b) tetramers to assess the numbers of HY-reactive CD8+ T cells in HYTCR-transgenic mice and in naive, wild-type C57BL/6 (B6) mice. As expected, tetramer staining showed the majority of T cells were male-specific CD8+ T cells in female HY-TCR mice. Staining of B6 mice showed a small population of male-specific CD8+ T cells in female mice. The effect of administration of soluble MHC class I tetramers on CD8+ T cell activation in vivo was unknown. Injection of HY-D(b) tetramer in vivo effectively primed female mice for a more rapid proliferative response to both HY peptide and male splenocytes. Furthermore, wild-type B6 female mice injected with a single dose of HY-D(b) tetramer rejected B6 male skin grafts more rapidly than female littermates treated with irrelevant tetramer. In contrast, multiple doses of HY-D(b) tetramer did not further decrease graft survival. Rather, female B6 mice injected with multiple doses of HY-D(b) tetramer rejected male skin grafts more slowly than mice primed with a single injection of tetramer or irradiated male spleen cells, suggesting clonal exhaustion or anergy. Our data highlight the ability of soluble MHC tetramers to identify scarce alloreactive T cell populations and the use of such tetramers to directly modulate an Ag-specific T cell response in vivo.

CD8(+) T cells express a T-helper 1--like phenotype after burn injury

BACKGROUND

Previous studies suggest that CD8(+) T cells are immunosuppressive after burn injury, but recent reports indicate that CD8(+) T cells have several functions similar to CD4(+) T cells, including the secretion of cytokines. This study uses HY male antigen in transgenic HY female mice to determine the antigen-specific response of activated CD8(+) T cells after burn injury.

METHODS

HY TCR transgenic female mice underwent burn or sham injury. Seventy-two hours after the burn, splenocytes were stimulated with 20 micromol/L HY peptide for 16, 48, and 64 hours; cellular proliferation, intracellular interferon-gamma and interleukin-2, and apoptosis were measured.

RESULTS

Burn injury significantly impaired proliferation to HY antigen (P < or =.05). Activated CD8(+) T cells from burned mice showed increased intracellular interferon-gamma and interleukin-2 16 hours after stimulation compared with sham (P < or =.05) and at no time was less than control mice. The percent of CD8(+) T cells decreased with the time of stimulation but was not due to apoptosis by Annexin V staining.

CONCLUSIONS

Activated CD8(+) T cells express a T(h1)-like phenotype after burn injury. This provides evidence that CD8(+) T cells are not simply suppressive and that is consistent with data that CD4(+) T cells are primed for a T(h1) response after burn injury.

CYP1A2 activity as measured by a caffeine test predicts clozapine and active metabolite steady-state concentrationin patients with schizophrenia

Clozapine is an atypical antipsychotic drug and displays efficacy in 30% to 60% of patients with schizophrenia who do not respond to traditional antipsychotics. A clozapine concentration greater than 1,150 nmol/L increases the probability of antipsychotic efficacy. However, plasma clozapine concentration can vary more than 45-fold during long-term treatment. The aim of this study was to assess the contribution of CYP1A2 to variability in steady-state concentration of clozapine and its active metabolite norclozapine. Patients with schizophrenia or schizoaffective disorder were prospectively monitored during clozapine treatment (N = 18). The in vivo CYP1A2 activity was measured using the caffeine metabolic ratio (CMR) in overnight urine. Trough plasma samples were drawn after at least 5 days of treatment with a constant regimen of clozapine. A significant negative association was found between the CMR and the dose-corrected clozapine (r(s) = -0.87,p < 0.01) and norclozapine (r(s) = -0.76,p < 0.01) concentrations. Nonsmokers displayed a higher clozapine (3.2-fold) and norclozapine (2.3-fold) concentration than smokers (p < 0.05). Furthermore, there was marked person-to-person variation in CYP1A2 activity during multiple-dose clozapine treatment (coefficient of variation = 60%). Age, weight, serum creatinine, and grapefruit juice consumption did not significantly contribute to variability in clozapine and norclozapine concentration (p > 0.05). In conclusion, CYP1A2 is one of the important contributors to disposition of clozapine during multiple-dose treatment. Although further in vitro experiments are necessary, the precise metabolic pathways catalyzed by CYP1A2 seem to be subsequent to the formation of norclozapine, hitherto less recognized quantitatively important alternate disposition routes, or both. From a clinical perspective, an environmentally induced or constitutively high CYP1A2 expression can lead to a decrease in steady-state concentration of clozapine as well as its active metabolite norclozapine. Thus, interindividual variability in CYP1A2 activity may potentially explain treatment resistance to clozapine in some patients. CYP1A2 phenotyping with a simple caffeine test may contribute to individualization of clozapine dosage and differentiate between treat ment noncompliance and high CYP1A2 activity.

Class I major histocompatibility complex anchor substitutions alter the conformation of T cell receptor contacts

An immunogenic peptide (GP2) derived from HER-2/neu binds to HLA-A2.1 very poorly. Some altered-peptide ligands (APL) of GP2 have increased binding affinity and generate improved cytotoxic T lymphocyte recognition of GP2-presenting tumor cells, but most do not. Increases in binding affinity of single-substitution APL are not additive in double-substitution APL. A common first assumption about peptide binding to class I major histocompatibility complex is that each residue binds independently. In addition, immunologists interested in immunotherapy frequently assume that anchor substitutions do not affect T cell receptor contact residues. However, the crystal structures of two GP2 APL show that the central residues change position depending on the identity of the anchor residue(s). Thus, it is clear that subtle changes in the identity of anchor residues may have significant effects on the positions of the T cell receptor contact residues.

Gorillas with spondyloarthropathies express an MHC class I molecule with only limited sequence similarity to HLA-B27 that binds peptides with arginine at P2

The human MHC class I gene, HLA-B27, is a strong risk factor for susceptibility to a group of disorders termed spondyloarthropathies (SpAs). HLA-B27-transgenic rodents develop SpAs, implicating HLA-B27 in the etiology of these disorders. Several nonhuman primates, including gorillas, develop signs of SpAs indistinguishable from clinical signs of humans with SpAs. To determine whether SpAs in gorillas have a similar HLA-B27-related etiology, we analyzed the MHC class I molecules expressed in four affected gorillas. Gogo-B01, isolated from three of the animals, has only limited similarity to HLA-B27 at the end of the alpha1 domain. It differs by several residues in the B pocket, including differences at positions 45 and 67. However, the molecular model of Gogo-B*0101 is consistent with a requirement for positively charged residues at the second amino acid of peptides bound by the MHC class I molecule. Indeed, the peptide binding motif and sequence of individual ligands eluted from Gogo-B*0101 demonstrate that, like HLA-B27, this gorilla MHC class I molecule binds peptides with arginine at the second amino acid position of peptides bound by the MHC class I molecule. Furthermore, live cell binding assays show that Gogo-B*0101 can bind HLA-B27 ligands. Therefore, although most gorillas that develop SpAs express an MHC class I molecule with striking differences to HLA-B27, this molecule binds peptides similar to those bound by HLA-B27.

Tumor-specific CD4+ T cells have a major "post-licensing" role in CTL mediated anti-tumor immunity

A number of tumor studies have indicated a link between CD4 help and the magnitude and persistence of CTL activity; however, the mechanisms underlying this have been largely unclear. To evaluate and determine the mechanisms by which CD4(+) T cells synergize with CD8(+) T cells to prevent tumor growth, we used the novel technique of monitoring in vivo CTL by labeling target cells with CFSE. This approach was supported by the direct visualization of CTL using peptide-MHC tetramers to follow tumor-specific T cells. The data presented demonstrate that while cotransfer of Ag-specific CD4(+) T cells was not required for the generation of CTLs, because adoptive transfer of CD8(+) T cells alone was sufficient, CD4(+) T cells were required for the maintenance of CD8(+) T cell numbers. Our data suggest that there is a correlation among the number of CD8(+) T cells, in vivo CTL function, and IFN-gamma production, with no evidence of a partial or nonresponsive phenotype among tetramer-positive cells. We also show that CD4(+) T cells are required for CD8(+) T cell infiltration of the tumor.

Quantitation of CD8(+) T-lymphocyte responses to multiple epitopes from simian virus 40 (SV40) large T antigen in C57BL/6 mice immunized with SV40, SV40 T-antigen-transformed cells, or vaccinia virus recombinants expressing full-length T antigen or epitope minigenes

The cytotoxic T-lymphocyte response to wild-type simian virus 40 large tumor antigen (Tag) in C57BL/6 (H2(b)) mice is directed against three H2-D(b)-restricted epitopes, I, II/III, and V, and one H2-K(b)-restricted epitope, IV. Epitopes I, II/III, and IV are immunodominant, while epitope V is immunorecessive. We investigated whether this hierarchical response was established in vivo or was due to differential expansion in vitro by using direct enumeration of CD8(+) T lymphocytes with Tag epitope/major histocompatibility complex class I tetramers and intracellular gamma interferon staining. The results demonstrate that epitope IV-specific CD8(+) T cells dominated the Tag-specific response in vivo following immunization with full-length Tag while CD8(+) T cells specific for epitopes I and II/III were detected at less than one-third of this level. The immunorecessive nature of epitope V was apparent in vivo, since epitope V-specific CD8(+) T cells were undetectable following immunization with full-length Tag. In contrast, high levels of epitope V-specific CD8(+) T lymphocytes were recruited in vivo following immunization and boosting with a Tag variant in which epitopes I, II/III, and IV had been inactivated. In addition, analysis of the T-cell receptor beta (TCRbeta) repertoire of Tag epitope-specific CD8(+) cells revealed that multiple TCRbeta variable regions were utilized for each epitope except Tag epitope II/III, which was limited to TCRbeta10 usage. These results indicate that the hierarchy of Tag epitope-specific CD8(+) T-cell responses is established in vivo.

Class I MHC is stabilized against thermal denaturation by physiological concentrations of NaCl

Class I MHC molecules are ternary complexes composed of an allotype specific heavy chain, a noncovalently associated protein beta(2)-microglobulin (beta(2)m), and a peptide. The complexes are assembled in the endoplasmic reticulum by a complex series of chaperones and peptide-loading mechanisms. In the absence of beta(2)m or peptide, very little class I heavy chain is transported to the surface of the cell. Complexes that do not contain all three parts of the protein are not made productively in vivo and not at all in vitro. The ability of the complex to withstand thermal denaturation in vitro has been shown to be related to the binding affinity of the peptide. Paradoxically, some low-affinity peptide complexes denature at or below human basal body temperatures in vitro but are effective biological agents in vivo. Here we show that these complexes are stabilized against thermal denaturation by physiological cosolvents and maximally stabilized by 150 mM NaCl. While the degree of stabilization by 150 mM NaCl is greatest for low-affinity peptide/MHC complexes, the mechanism of stabilization is independent of peptide sequence. This effect is hypothesized to occur by multiple mechanisms including increasing the affinity of beta(2)m for the complex and charge screening.

Correlated switch binding and signaling in bacterial chemotaxis

In Escherichia coli, swimming behavior is mediated by the phosphorylation state of the response regulator CheY. In its active, phosphorylated form, CheY exhibits enhanced binding to a switch component, FliM, at the flagellar motor, which induces a change from counterclockwise to clockwise flagellar rotation. When Ile(95) of CheY is replaced by a valine, increased clockwise rotation correlates with enhanced binding to FliM. A possible explanation for the hyperactivity of this mutant is that residue 95 affects the conformation of nearby residues that potentially interact with FliM. In order to assess this possibility directly, the crystal structure of CheY95IV was determined. We found that CheY95IV is structurally almost indistinguishable from wild-type CheY. Several other mutants with substitutions at position 95 were characterized to establish the structural requirements for switch binding and clockwise signaling at this position and to investigate a general relationship between the two properties. The various rotational phenotypes of these mutants can be explained solely by the amount of phosphorylated CheY bound to the switch, which was inferred from the phosphorylation properties of the mutant CheY proteins and their binding affinities to FliM. Combined genetic, biochemical, and crystallographic results suggest that residue 95 itself is critical in mediating the surface complementarity between CheY and FliM.

Peptide binding by class I and class II MHC molecules

Major histocompatibility complex (MHC) antigens bind peptides of diverse sequences with high affinity. They do this in order to generate maximal immunological protection by covering the spectrum of peptides that may be seen by a host over the course of its lifetime. However, in many circumstances the immune system does not recognize a particular peptide that it should for maximum advantage over the pathogen. In other situations, the immune system goes awry and incorrectly recognizes a self-peptide that it should not. This results in disease characterized by recognition and attack of self. Rheumatoid arthritis is an example of just such a disease. In either of these situations, peptide-based modalities for immune therapy would be an advantage. However, peptide-based therapies require a thorough understanding of the forces involved in peptide binding. Great strides have been made in elucidating the mechanisms by which these MHC proteins may bind peptides with diverse sequences and high affinity. This review summarizes the current data obtained from crystallographic analyses of peptide binding for both class I and class II MHC molecules. Unfortunately, as yet these data have not allowed us to predict which peptides will bind with high affinity to a specific MHC molecule.

T cell activity after dendritic cell vaccination is dependent on both the type of antigen and the mode of delivery

Previous work in both human and animal models has shown that CTL responses can be generated against proteins derived from tumors using either peptide-pulsed dendritic cells (DCs) or nucleic acids from the tumor transfected into autologous DCs. Despite the efficacy of this approach for vaccine therapy, many questions remain regarding whether the route of administration, the frequency of administration, or the type of Ag is critical to generating T cell responses to these Ags. We have investigated methods to enhance CTL responses to a peptide derived from the human proto-oncogene HER-2/neu using mice containing a chimeric HLA A2 and H2Kb allele. Changes in amino acids in the anchor positions of the peptide enhanced the binding of the peptide to HLA-A2 in vitro, but did not enhance the immunogenicity of the peptide in vivo. In contrast, when autologous DCs presented peptides, significant CTL activity was induced with the altered, but not the wild-type, peptide. We found that the route of administration affected the anatomic site and the time to onset of CTL activity, but did not impact on the magnitude of the response. To our surprise, we observed that weekly administration of peptide-pulsed DCs led to diminishing CTL activity after 6 wk of treatment. This was not found in animals injected with DCs every 3 wk for six treatments or in animals initially given DCs weekly and then injected weekly with peptide-pulsed C1R-A2 transfectants.

Naive CD8+ T cells do not require costimulation for proliferation and differentiation into cytotoxic effector cells

Most current models of T cell activation postulate a requirement for two distinct signals. One signal is delivered through the TCR by engagement with peptide/MHC complexes, and the second is delivered by interaction between costimulatory molecules such as CD28 and its ligands CD80 and CD86. Soluble peptide/MHC tetramers provide an opportunity to test whether naive CD8+ T cells can be activated via the signal generated through the TCR-alphabeta in the absence of any potential costimulatory molecules. Using T cells from two different TCR transgenic mice in vitro, we find that TCR engagement by peptide/MHC tetramers is sufficient for the activation of naive CD8+ T cells. Furthermore, these T cells proliferate, produce cytokines, and differentiate into cytolytic effectors. Under the conditions where anti-CD28 is able to enhance proliferation of normal B6 CD4+, CD8+, and TCR transgenic CD8+ T cells with anti-CD3, we see no effect of anti-CD28 on proliferation induced by tetramers. The results of this experiment argue that given a strong signal delivered through the TCR by an authentic ligand, no costimulation is required.

The structural basis for the increased immunogenicity of two HIV-reverse transcriptase peptide variant/class I major histocompatibility complexes

Designing altered peptide ligands to generate specific immunological reactivity when bound to class I major histocompatibility complexes is important for both therapeutic and prophylactic reasons. We have previously shown that two altered peptides, derived from human immunodeficiency virus (HIV)-reverse transcriptase (RT) residues 309-317, are more immunogenic in vitro than the wild-type peptide. One peptide variant, I1Y, was able to stimulate RT-specific cytotoxic T cells from the blood of three HIV-infected individuals better than the wild-type RT peptide. Both I1Y and I1F peptide variants increase the cell surface half-life of the peptide-class I complex approximately 3-fold over that of the RT peptide but have different immunological activities. These peptides are candidates for the design of vaccines for HIV due to their increased immunogenicity. To understand the basis for the increased cell surface stability compared with wild-type peptide and to understand the differences in T cell recognition between I1Y and I1F, we determined the x-ray crystal structures of the two class I MHC-peptide complexes. These structures indicate that the increased cell surface half-life is due to pi-pi stacking interactions between Trp-167 of HLA-A2.1 and the aromatic P1 residues of I1F and I1Y. Comparison of the structures and modeling potential T cell receptor (TCR) interactions suggests that T cell interactions and immunogenicity are different between I1Y and I1F for two reasons. First, subtle changes in the steric and polar properties of the I1Y peptide affect TCR engagement. Second, water-mediated hydrogen bond interactions between the P1-Tyr and the P4-Glu peptide residues increase peptide side chain rigidity of residues critical for TCR engagement.

Poor binding of a HER-2/neu epitope (GP2) to HLA-A2.1 is due to a lack of interactions with the center of the peptide

Class I major histocompatibility complex (MHC) molecules bind short peptides derived from proteins synthesized within the cell. These complexes of peptide and class I MHC (pMHC) are transported from the endoplasmic reticulum to the cell surface. If a clonotypic T cell receptor expressed on a circulating T cell binds to the pMHC complex, the cell presenting the pMHC is killed. In this manner, some tumor cells expressing aberrant proteins are recognized and removed by the immune system. However, not all tumors are recognized efficiently. One reason hypothesized for poor T cell recognition of tumor-associated peptides is poor binding of those peptides to class I MHC molecules. Many peptides, derived from the proto-oncogene HER-2/neu have been shown to be recognized by cytotoxic T cells derived from HLA-A2(+) patients with breast cancer and other adenocarcinomas. Seven of these peptides were found to bind with intermediate to poor affinity. In particular, GP2 (HER-2/neu residues 654-662) binds very poorly even though it is predicted to bind well based upon the presence of the correct HLA-A2.1 peptide-binding motif. Altering the anchor residues to those most favored by HLA-A2.1 did not significantly improve binding affinity. The crystallographic structure shows that unlike other class I-peptide structures, the center of the peptide does not assume one specific conformation and does not make stabilizing contacts with the peptide-binding cleft.

Structural evidence of T cell xeno-reactivity in the absence of molecular mimicry

The T cell receptor (TCR), from a xeno-reactive murine cytotoxic T lymphocyte clone AHIII12.2, recognizes murine H-2Db complexed with peptide p1027 (FAPGVFPYM), as well as human HLA-A2.1 complexed with peptide p1049 (ALWGFFPVL). A commonly proposed model (the molecular mimicry model) used to explain TCR cross-reactivity suggests that the molecular surfaces of the recognized complexes are similar in shape, charge, or both, in spite of the primary sequence differences. To examine the mechanism of xeno-reactivity of AHIII12.2, we have determined the crystal structures of A2/p1049 and Db/p1027 to 2.5 A and 2.8 A resolution, respectively. The crystal structures show that the TCR footprint regions of the two class I complexes are significantly different in shape and charge. We propose that rather than simple molecular mimicry, unpredictable arrays of common and differential contacts on the two class I complexes are used for their recognition by the same TCR.

Extensive alanine substitutions increase binding affinity of an influenza nucleoprotein peptide to HLA-Aw68 and do not abrogate peptide-specific CTL recognition

Class I MHC molecules bind peptides in the endoplasmic reticulum and present them at the cell surface to circulating CD8+ T cells for analysis. We have examined binding of peptides and stabilization of HLA-Aw68 class I molecules using synthetic peptide variants of an influenza virus nucleoprotein peptide, NP91-99 (KTGGPIYKR). We have demonstrated that insertion of increasing numbers of alanines in the center of the peptide (between P and I), to increase a natural bulging out of the peptide-binding cleft, results in a large decrease in thermal stability. Although there is a great decrease in the t 1/2 of the MHC/peptide complex for NP-1A compared with NP91-99, a T cell line, stimulated by NP91-99, recognizes NP-1A efficiently. Peptide variants with three or more alanines do not show saturable binding to HLA-Aw68 and also are not recognized by the T cell line. Thermal studies show that polyalanine peptides with minimal anchors and nearly all TCR contact residues exchanged stabilized HLA-Aw68 to high temperatures. Additionally, some of these polyalanine peptides are recognized by T cell lines generated against NP91-99. Analysis of the peptide sequences show that the stabilization effects are not due to the hydrophobicity of the peptide. These data suggest that the strength of binding of peptides to HLA-Aw68 is not only dictated by the presence of anchor residues but also by the lack of unfavorable contacts between the peptide ligand and class I MHC-binding cleft.

Altered peptide ligand design: altering immune responses to class I MHC/peptide complexes

Class I proteins are responsible for binding proteins from endogenously synthesized proteins and displaying them on the cell surface. Our understanding of this process has reached the point where we can manipulate the biochemical properties of peptide/class I binding and determine the effects of this alteration on subsequent immune responses. In this article, we will review the biochemistry of peptide/class I binding, and the effects of structure on this interaction between class I proteins and their peptide ligands. We will review the data which suggest that the major relevant biochemical parameter of class I peptide binding is the off-rate. We will show that the design of altered ligands with improved binding, thermostability and immunogenicity is possible.

Diminished class II-associated Ii peptide binding to the juvenile dermatomyositis HLA-DQ alpha 1*0501/DQ beta 1*0301 molecule

HLA class II molecules bind and present peptide Ags to T cells, binding specific sets of peptides due to polymorphism in the peptide binding groove. Class II proteins associate with the invariant chain (Ii chain) and its derived class II-associated Ii peptide (CLIP). Ii chain association is important for normal trafficking of class II proteins to the peptide loading vesicles and for blocking premature access of peptides to HLA class II molecules during maturation. We have previously shown that juvenile dermatomyositis is associated with the HLA-DQA1*0501 allele. There is limited information available about the interaction of any DQ molecule with the Ii chain and little information about binding of individual peptides to HLA-DQalpha1*0501/DQbeta1*0301. We sequenced peptides eluted from the juvenile dermatomyositis-associated class II allele HLA-DQalpha1*0501/DQbeta1*0301. Surprisingly, we found no Ii chain or CLIP. Further examination of peptide binding to the HLA-DQalpha1*0501/DQbeta1*0301 molecule demonstrated poor CLIP binding. However, newly synthesized HLA-DQalpha1*0501/DQbeta1*0301 molecules do associate with intact Ii chain. Molecular modeling suggests that CLIP binds differently to HLA-DQalpha1*0501/DQbeta1*0301 than to DR molecules. The lack of CLIP association suggests that HLA-DQalpha1*0501/DQbeta1*0301 has access to peptides earlier in the processing pathway and so might encounter novel peptides that induce autoimmunity.

Diminished class II-associated Ii peptide binding to the juvenile dermatomyositis HLA-DQ alpha 1*0501/DQ beta 1*0301 molecule

HLA class II molecules bind and present peptide Ags to T cells, binding specific sets of peptides due to polymorphism in the peptide binding groove. Class II proteins associate with the invariant chain (Ii chain) and its derived class II-associated Ii peptide (CLIP). Ii chain association is important for normal trafficking of class II proteins to the peptide loading vesicles and for blocking premature access of peptides to HLA class II molecules during maturation. We have previously shown that juvenile dermatomyositis is associated with the HLA-DQA1*0501 allele. There is limited information available about the interaction of any DQ molecule with the Ii chain and little information about binding of individual peptides to HLA-DQalpha1*0501/DQbeta1*0301. We sequenced peptides eluted from the juvenile dermatomyositis-associated class II allele HLA-DQalpha1*0501/DQbeta1*0301. Surprisingly, we found no Ii chain or CLIP. Further examination of peptide binding to the HLA-DQalpha1*0501/DQbeta1*0301 molecule demonstrated poor CLIP binding. However, newly synthesized HLA-DQalpha1*0501/DQbeta1*0301 molecules do associate with intact Ii chain. Molecular modeling suggests that CLIP binds differently to HLA-DQalpha1*0501/DQbeta1*0301 than to DR molecules. The lack of CLIP association suggests that HLA-DQalpha1*0501/DQbeta1*0301 has access to peptides earlier in the processing pathway and so might encounter novel peptides that induce autoimmunity.

A general technique for computing evolutionarily stable strategies based on errors in decision-making

Realistic models of contests between animals will often involve a series of state-dependent decisions by the contestants. Computation of evolutionarily stable strategies for such state-dependent dynamic games are usually based on damped iterations of the best response map. Typically this map is discontinuous so that iterations may not converge and even if they do converge it may not be clear if the limiting strategy is a Nash equilibrium. We present a general computational technique based on errors in decision making that removes these computational difficulties. We show that the computational technique works for a simple example (the Hawk-Dove game) where an analytic solution is known, and prove general results about the technique for more complex games. It is also argued that there is biological justification for inclusion of the types of errors we have introduced.

Covalent HLA-B27/peptide complex induced by specific recognition of an aziridine mimic of arginine

The class I major histocompatibility complex (MHC) glycoprotein HLA-B27 binds short peptides containing arginine at peptide position 2 (P2). The HLA-B27/peptide complex is recognized by T cells both as part of the development of the repertoire of T cells in the cellular immune system and during activation of cytotoxic T cells. Based on the three-dimensional structure of HLA-B27, we have synthesized a ligand with an aziridine-containing side chain designed to mimic arginine and to bind covalently in the arginine-specific P2 pocket of HLA-B27. Using tryptic digestion followed by mass spectrometry and amino acid sequencing, the aziridine-containing ligand is shown to alkylate specifically cysteine 67 of HLA-B27. Neither free cysteine in solution nor an exposed cysteine on a class II MHC molecule can be alkylated, showing that specific recognition between the anchor side-chain pocket of an MHC class I protein and the designed ligand (propinquity) is necessary to induce the selective covalent reaction with the MHC class I molecule.

Clozapine: current status and role in the pharmacotherapy of schizophrenia

OBJECTIVE

This study evaluates clozapine and its present role in the pharmacotherapy of schizophrenia.

METHOD

Clozapine's current clinical status is reviewed, as is its position with respect to other treatment options.

RESULTS

Clozapine represents the prototype of "atypical" neuroleptics, with evidence of clinical efficacy in both positive and negative symptoms, as well as a diminished risk of extrapyramidal side effects. It is the only neuroleptic to date that has established itself as having little, if any, risk of tardive dyskinesia. More recent research has focused on its potential for overall savings in health care costs, as well as possible benefits in the area of neuropsychological functioning.

CONCLUSION

Evidence suggesting that the course of schizophrenia can be altered by effective treatment favours a systematic approach that optimizes treatment options. While clozapine does not represent a 1st-line agent because of its risk of agranulocytosis, it has an integral role to play in treatment-resistant schizophrenia or in individuals experiencing intolerable side effects with conventional neuroleptics.

A tricyclic ring system replaces the variable regions of peptides presented by three alleles of human MHC class I molecules

BACKGROUND

Cytotoxic T-lymphocytes (CTLs) recognize complexes of short peptides with major histocompatibility complex (MHC) class I molecules. MHC molecules are polymorphic, and the products of different MHC alleles bind to different subsets of peptides. This is due to differences in the shape of the peptide-binding groove on the surface of the MHC protein, especially the 'pockets' into which anchor residues at each end of the peptide fit. Non-peptidic ligands for class I molecules may be useful clinically.

RESULTS

By applying computer-aided design methods guided by X-ray structures, we designed and synthesized several MHC class I ligands, based on known peptide ligands, in which the tricyclic, aromatic compound phenanthridine replaced the central amino acids of the peptides. These semi-peptidic fluorescent ligands bound with high affinity and with allelic specificity to the peptide-binding groove of different MHC class I molecules, forming crystallizable complexes.

CONCLUSIONS

Specificity for binding to different MHC class I molecules can be imparted to the common phenanthridine element by judicious choice of terminal peptidic elements from either nonamer or decamer peptides. The phenanthridine-based ligands have a long bound half-life, as do antigenic peptides.

Clinical use of clozapine in a major urban setting: one year experience

This paper examines the clinical and demographic data of patients in the Clozapine Distribution System in Metropolitan Toronto in the first year after its inception. One hundred and thirty-seven patients were approved for funding during the year. They tended to be young, chronically and markedly ill patients suffering from schizophrenia, primarily with treatment resistance as the reason for clozapine therapy. Only 55 patients completed at least six months of therapy; 15 patients discontinued clozapine before six months of treatment, mainly because of side-effects and/or patients' noncompliance with bloodwork. Three patients discontinued clozapine because of haematological compromise. Clozapine was efficacious for the majority of patients who took it for at least six months, with improvement in all six clinical dimensions examined in the study. Nevertheless, the number of early discontinuation patients significantly lowered the overall effectiveness of clozapine in actual clinical practice.

The three-dimensional structure of a class I major histocompatibility complex molecule missing the alpha 3 domain of the heavy chain

Class I major histocompatibility complex (MHC) molecules are ternary complexes of the soluble serum protein beta 2-microglobulin, MHC heavy chain, and bound peptide. The first two domains (alpha 1, alpha 2) of the heavy chain create the peptide binding cleft and the surface that contacts the T-cell receptor. The third domain (alpha 3) associates with the T-cell co-receptor, CD8, during T-cell recognition. Here we describe the x-ray crystal structure of a human class I MHC molecule, HLA-Aw68, from which the alpha 3 domain has been proteolytically removed. The resulting molecule shows no gross morphological changes compared to the intact protein. A decameric peptide complexed with the intact HLA-Aw68 is seen to bind to the proteolized molecule in the conventional manner, demonstrating that the alpha 3 domain is not required for the structural integrity of the molecule or for peptide binding.

Three-dimensional structure of a peptide extending from one end of a class I MHC binding site

Class I major histocompatibility complex (MHC) molecules present peptides to CD8+ T cells for immunological surveillance (reviewed in ref. 1). The structures of complexes of class I MHC molecules with octamer, nonamer and decamer peptides determined until now show a common binding mode, with both peptide termini bound in conserved pockets at the ends of the peptide binding site. Length variations were accommodated by the peptide bulging or zig-zagging in the middle. Here we describe the structure of a decamer peptide which binds with the carboxy-terminal residue positioned outside the peptide binding site. Several protein side chains have rearranged to allow the peptide to exit. The structure suggests that even longer peptides could bind. The energetic effect of the altered mode of binding has been assessed by measuring the stability of the complex to thermal denaturation. Peptides bound in this novel manner are stable at physiological temperature, raising questions about their role in T-cell recognition and their production by proteolytic processing.

Positive and negative symptoms in families with schizophrenia

Positive and negative symptoms are measurable characteristics that may represent core features of schizophrenia and offer a quantitative approach for studying the genetics of schizophrenia and related disorders. The Positive and Negative Syndrome Scale (PANSS) was used to assess 72 members of five families segregating schizophrenia. The study confirmed high internal reliability of PANSS scales in this sample with diverse lifetime diagnoses. Gender but not alcoholism affected scores. Schizophrenia/schizoaffective and schizophrenia spectrum disorder groups had higher mean scores for the positive and negative scales than other lifetime diagnostic groups, consistent with genetic transmission of these symptoms. Positive and negative symptom patterns did not subtype families. The results support the validity of positive and negative symptom measures as independent dimensions in familial schizophrenia.

The 2.5 A structure of pokeweed antiviral protein

The pokeweed antiviral protein (PAP), isolated from the leaves of Phytolacca americana, is one of a family of plant and bacterial ribosome-inhibiting proteins (RIPs) which act as specific N-glycosidases on rRNA. Here we report the three-dimensional structure of PAP determined to 2.5 A resolution by X-ray crystallography. After 14 rounds of refinement, the R factor is 0.17 for 5.0 to 2.5 A data. The protein is homologous with the A chain of ricin and exhibits a very similar folding pattern. The positions of key active site residues are also similar. We also report the 2.8 A structure of PAP complexed with a substrate analog, formycin 5'-monophosphate. As seen previously in ricin, the formycin ring is stacked between invariant tyrosines 72 and 123. Arg179 bonds to N-3 which is thought to be important in catalysis.

Clozapine in the treatment of refractory schizophrenia: Canadian policies and clinical guidelines

Clozapine is an atypical neuroleptic agent that has recently become available in Canada with potential clinical efficacy in the treatment of refractory schizophrenia, and in patients with schizophrenia neurologically intolerant to conventional neuroleptics. Although it causes few extra-pyramidal symptoms, the drug has a number of other adverse effects including a risk of agranulocytosis in one to two percent of all patients. Because of this, the use of the drug is permitted only if the white blood count is monitored weekly. The monitoring system, outlined in this article, requires a coordinated effort between clinical staff, pharmacy, laboratory and the Clozaril Support and Assistance Network. Clinical guidelines are proposed, detailing the indications and contraindications for treatment and the pharmacokinetics, dosing, adverse effects, and drug interactions with clozapine. In addition, the economics, government policies and implications for future research are considered. Although there are administrative and clinical difficulties associated with its use, clozapine represents an advance in therapeutic research. Patients and family members will be inquiring about the drug and may deserve a trial. This article aims to inform Canadian mental health professionals about the safe and beneficial use of clozapine.

The pharmacoepidemiology of treatment-refractory schizophrenia

Treatment-refractory schizophrenia is a major clinical problem for which there is relatively little scientific information, and no consensus has been reached on approaches to treatment. The pharmacotherapy used for 103 patients with a diagnosis of schizophrenia or schizoaffective disorder at one psychiatric hospital was examined. Data were gathered on neuroleptic choice and dose, the use of adjunctive treatments and serum neuroleptic levels. The daily neuroleptic dose was compared with that of random samples of patients receiving outpatient maintenance treatment, and short-stay patients receiving acute treatment. The patients in our sample received high doses of neuroleptics despite a persistent lack of response to treatment, and despite the fact that these were in excess of the recommended maximum beneficial dose. The appropriateness of this therapy is discussed.

Measurement of therapeutic response in schizophrenia. A critical survey

A recurrent criticism of measurement in schizophrenia research is that symptom suppression is overemphasized as the sole criterion measure of treatment effectiveness, to the neglect of other endpoints, such as the quality of life and subjective experience of the patient. This paper addresses the current status of response measures used in schizophrenia therapeutics. A computer literature search of all clinical trials (drug, psychosocial, or rehabilitative) in schizophrenia for the years 1986 through 1989 yielded 175 papers, 79 of which were trials of therapy. Almost all the trials focused on productive symptoms, with 19% considering negative symptoms scales as well. Approximately one-quarter of the studies incorporated some measure of patient functioning, but only 13% considered the patient's self-report or ratings of significant others. The findings suggest that the measurement of treatment response in schizophrenia research continues to be unidimensional and symptom-based. It is proposed that treatment effectiveness may be obscured when measures of patient functioning, subjective experience and assessments of significant others are not included with those of symptomatology.

Structure of ricin A-chain at 2.5 A

Ricin has been refined in a crystallographic sense to 2.5 A resolution and the model for the A-chain (RTA) is described in detail. Because RTA is the first member of the class of plant toxins to be analyzed, this model probably defines the major structural characteristics of the entire family of these medically important proteins. Explanations are provided to rationalize amino acids that are conserved between RTA and a number of homologous plant and bacterial toxins. Eight invariant residues appear to be involved in creating or stabilizing the active site. In the active site Arg180 and Glu177 are hydrogen bonded to each other and also coordinate a water molecule; each of these groups may be important in the N-glycosidation reaction. Several other polar residues may play lesser roles in the mechanism, including tyrosines 80 and 123 and asparagines 78 and 209. A number of conserved hydrophobic residues are seen to cluster within several patches and probably drive the overall folding of the toxin molecule.

Crystallographic refinement of ricin to 2.5 A

The plant cytotoxin ricin consists of two disulfide-linked chains, each of about 30,000 daltons. An initial model based on a 2.8 A MIR electron density map has been refined against 2.5 A data using rounds of hand rebuilding coupled with either a restrained least squares algorithm or molecular dynamics (XPLOR). The last model (9) has an R factor of 21.6% and RMS deviations from standard bond lengths and angles of 0.021 A and 4.67 degrees, respectively. Refinement required several peptide segments in the original model to be adjusted translationally along the electron density. A wide range of lesser changes were also made. The RMS deviation of backbone atoms between the original and model 9 was 1.89 A. Molecular dynamics proved to be a very powerful refinement tool. However, tests showed that it could not replace human intervention in making adjustments such as local translations of the peptide chain. The R factor is not a completely satisfactory indicator of refinement progress; difference Fouriers, when observed carefully, may be a better monitor.

Primary amino acid sequence of alpha-trichosanthin and molecular models for abrin A-chain and alpha-trichosanthin

Ricin A-chain, abrin A-chain, and alpha-trichosanthin are members of a larger group of proteins called ribosome-inactivating proteins. These proteins all function to catalytically inactivate eukaryotic 60 S ribosomal subunits leading to rapid shutdown of protein synthesis. They are homologous in sequence and are probably evolutionarily related. We have determined the complete primary amino acid sequence of alpha-trichosanthin and have found it to be homologous, as expected, to that of abrin A-chain and ricin A-chain. A crystal structure for ricin, which includes ricin A-chain and ricin B-chain, has been determined from x-ray diffraction data. Based on the sequence homologies of these proteins, we fit the primary sequences of abrin A-chain and alpha-trichosanthin to the backbone structure for ricin A-chain and have generated energy-minimized molecular models for them. These models should prove useful in studying the structural-functional relationships of these proteins in particular and of the class in general.

Cell-mediated immunity to hepatitis B virus antigens in mice: correlation of in vivo and in vitro assays

Cell mediated immunity (CMI) to hepatitis B viral antigens was studied in BALB/mice after immunization with purified hepatitis B surface antigen (HBsAg), or core antigen (HBcAg), with adjuvants. The two in vitro assays for cell-mediated immunity (CMI), utilizing lymph node cells, were release of interferon after exposure to antigen, and blast transformation of lymphocytes, and the in vivo assay was ear swelling at 24 h after local injection of antigen. Immunization with HBsAg or HBcAg with adjuvants induced antigen-specific cutaneous reactivity; if no adjuvants were given, immunization with HBcAg, but not HBsAg, induced cutaneous reactivity. CMI could be adoptively transferred by lymph node cells, but for only a limited period after immunization with HbsAg or HBcAg. The ability of lymph node cells from mice immunized with HBV antigens to transfer adoptively CMI correlated well with their production of interferon after challenge with antigen in vitro, but less well with blastogenesis after challenge with antigen in vitro, or with cutaneous reactivity to antigen in the donor mouse. Reliable antigen-specific lymphokine release assays, rather than blast transformation of lymphocytes or cutaneous reactivity after antigen challenge, are required to assess CMI to HBV antigens in the mouse and, by inference, in man.

Assessment of delayed-type hypersensitivity in man: a comparison of the "Multitest" and conventional intradermal injection of six antigens

Recall of delayed type hypersensitivity (DTH) as a test for cell-mediated immune competence was assessed in 254 subjects using the Multitest device which delivers seven skin-test antigens intradermally; 77 subjects were tested concurrently by Multitest and a conventional panel of six antigens. Similar results were obtained with Multitest and the conventional panel (R = 0.65). Reproducibility of Multitest between three observers, who independently assessed the aggregate size of reactions (the reaction score) in 45 subjects, was high (R = 0.89). Twenty-four subjects were tested twice 3 months apart; the correlation for the reaction score was high (R = 0.88), demonstrating the suitability of Multitest for serial studies of immune function. Anergy was infrequent (1%) among 110 healthy male controls but was more frequent (8%) among a group of 101 healthy male homosexuals (P less than 0.05). The response rate to particular test antigens differed for the three Australian groups tested and a previously studied French group. Hence there is a need to establish normal profiles of DTH responsiveness for different geographic areas, as well as among subjects of known age and sex, when assessing cell-mediated immunity by the level of DTH responsiveness to multiple skin test antigens.

Granular myoblastoma: perineal neoplasm of probable neural origin

A seventy-year-old man with a painless, perineal neoplasm was found to have a granular cell myoblastoma densely adherent to the bulbar urethra. This case represents the fourth known perineal neoplasm of neural origin.