Conditional PD-1/PD-L1 Probody Therapeutics Induce Comparable Antitumor Immunity but Reduced Systemic Toxicity Compared with Traditional Anti-PD-1/PD-L1 Agents

Immune-checkpoint blockade has revolutionized cancer treatment. However, most patients do not respond to single-agent therapy. Combining checkpoint inhibitors with other immune-stimulating agents increases both efficacy and toxicity due to systemic T-cell activation. Protease-activatable antibody prodrugs, known as Probody therapeutics (Pb-Tx), localize antibody activity by attenuating capacity to bind antigen until protease activation in the tumor microenvironment. Herein, we show that systemic administration of anti-programmed cell death ligand 1 (anti-PD-L1) and anti-programmed cell death protein 1 (anti-PD-1) Pb-Tx to tumor-bearing mice elicited antitumor activity similar to that of traditional PD-1/PD-L1-targeted antibodies. Pb-Tx exhibited reduced systemic activity and an improved nonclinical safety profile, with markedly reduced target occupancy on peripheral T cells and reduced incidence of early-onset autoimmune diabetes in nonobese diabetic mice. Our results confirm that localized PD-1/PD-L1 inhibition by Pb-Tx can elicit robust antitumor immunity and minimize systemic immune-mediated toxicity. These data provide further preclinical rationale to support the ongoing development of the anti-PD-L1 Pb-Tx CX-072, which is currently in clinical trials.

Abstract A164: EGFR-CD3 bispecific Probody™ therapeutic induces tumor regressions and increases maximum tolerated dose >60-fold in preclinical studies

T cell-engaging bispecific antibodies (TCBs) are highly potent therapeutics that direct the activity of cytotoxic T cells to tumors. TCBs have shown clinical activity in hematologic malignancies, but development of TCBs for solid tumor indications is proving more challenging. Due to their high potency, TCBs can target normal tissues with low antigen expression, resulting in significant on-target, off-tumor toxicity that can limit dosing to low levels. As a result, it has been difficult to reach the level of drug exposure required for efficacy without excessive toxicity. Therefore, novel methods are needed to enable the potent antitumor activity of TCBs while minimizing toxicity due to cytokine release and damage to healthy tissues. CytomX has developed a new class of recombinant, proteolytically activated antibody prodrugs (ProbodyTM therapeutics) that are “masked” to prevent binding to antigen in healthy tissue, but can become “unmasked” by proteases that are preferentially activated in the tumor microenvironment. In this way, Probody therapeutics are designed to increase therapeutic index by maximizing efficacy and minimizing on-target toxicity in normal tissues. Here we describe a T cell-engaging Bispecific Probody therapeutic (Pb-TCB) targeting Epidermal Growth Factor Receptor (EGFR) and CD3 that has been optimized for affinity, effector function, masking, and cleavability. In vitro, under protease-deficient conditions, we demonstrate that the unmasked EGFR-CD3 TCB has potent, EGFR-dependent tumor cell killing, while the doubly-masked EGFR-CD3 Pb-TCB reduces target-dependent cytotoxicity by more than 100,000-fold. However, in established tumor models where tumor-resident proteases are expected to be active, we demonstrate that Pb-TCBs potently induce tumor regressions. In nonhuman primates, the maximum tolerated dose (MTD) of the EGFR-CD3 Pb-TCB is more than 60-fold higher than the MTD of the unmasked TCB, and the tolerated exposure (AUC) is more than 10,000-fold higher. Finally, despite the 60-fold dose differential at the MTDs, transient serum cytokine and AST/ALT increases observed in nonhuman primates treated with the Pb-TCB are still lower than those induced by the TCB. By localizing activity to the tumor microenvironment, Pb-TCBs have the potential to expand clinical opportunities for T cell-engaging bispecific therapies that are limited by on-target toxicities, especially in solid tumors. Moreover, an EGFR-CD3 Pb-TCB has the potential to address EGFR-expressing tumors that are poorly responsive to existing EGFR-directed therapies. PROBODY is a trademark of CytomX Therapeutics, Inc.

Citation Format: Leila M. Boustany, Laurie Wong, Clayton W. White, Linnea Diep, Yuanhui Huang, Shouchun Liu, Jennifer H. Richardson, W. Michael Kavanaugh, Bryan A. Irving. EGFR-CD3 bispecific Probody™ therapeutic induces tumor regressions and increases maximum tolerated dose >60-fold in preclinical studies [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A164.

Field Evaluation of Indoor Thermal Fog and Ultra-Low Volume Applications For Control of Aedes aegypti in Thailand

Efficacies of a handheld thermal fogger (Patriot™) and a backpack ultra-low volume (ULV) sprayer (Twister™) with combinations of 2 different adulticides (pyrethrin, deltamethrin) and an insect growth regulator (pyriproxyfen) were field-tested and compared for their impact on reducing indoor Aedes aegypti populations in Thailand. The effectiveness of the indoor space sprays was evaluated by sampling the natural Ae. aegypti population in houses and determining their physiological status, by monitoring mortality of sentinel caged mosquitoes (AFRIMS strain) and by assessing larval mortality in laboratory bioassays using water exposed to the spray. A total of 14,742 Ae. aegypti were collected from Biogents Sentinel traps in this study. The combination of ULD® BP-300 (3% pyrethrin) and NyGuard® (10% pyriproxyfen) sprayed either by the Patriot or Twister significantly reduced some Ae. aegypti populations up to 20 days postspray relative to the control clusters. The addition of pyriproxyfen to the adulticide extended how long household mosquito populations were suppressed. In 2 of the 4 products being compared, the Twister resulted in higher mortality of caged mosquitoes compared with the Patriot. However, neither machine was able to achieve high mortality among Ae. aegypti placed in hidden (protected) cages. The larval bioassay results demonstrated that the Twister ULV provided better adult emergence inhibition than the Patriot (thermal fogger), likely due to larger droplet size.

Abstract 3211: PD-1-targeted Probody therapeutics provide anti-tumor efficacy and a 10-fold dose protection against systemic autoimmunity in preclinical studies

Immuno-oncology approaches have transformed the treatment of advanced melanoma, NSCLC and an increasing number of other cancers. Antibodies targeting the T cell checkpoint molecules CTLA-4 and PD-1 can reverse tumor suppression of T cell activation that prevents effective anti-tumor immunity. However, blockade of checkpoint molecules, particularly in combination, in non-tumor tissues can result in autoimmune side effects that can limit therapeutic utility. Combination of PD-1 and CTLA-4 blockade provides increased response rates but with concomitant increases in immune-related adverse events. Here we demonstrate that Probody™ therapeutics targeting PD-1 synergize with a CTLA-4 checkpoint inhibitor to eradicate established tumors in a mouse model while protecting against PD1-mediated systemic autoimmunity.

Probody therapeutics are recombinant, proteolytically-activated antibody prodrugs that have the potential to meaningfully widen therapeutic index by minimizing interactions with normal tissue while retaining anti-tumor activity. Probody therapeutics take advantage of protease activities that are abundant in tumors but suppressed in normal tissues. An extension of the antibody light chain with a masking peptide blocks binding of the antibody to antigen in normal tissue. Tumor-associated proteases cleave and release the mask, enabling the active antibody to bind antigen preferentially in the tumor. The feasibility of the Probody approach to checkpoint targets is supported by preclinical and clinical studies in which intratumoral delivery of low dose immune modulators enables anti-tumor responses despite negligible systemic exposure.

For preclinical assessment of PD-1 as a Probody target, a family of Probody therapeutics derived from the anti-mouse PD-1 antibody J43 was developed. The intact Probody therapeutics demonstrated reduced binding to mouse PD-1 relative to the parental antibody that was completely restored following proteolytic activation. The anti-tumor efficacy of the J43 antibody and Probody therapeutics was assessed following systemic administration as single agents or in combination with an anti-CTLA-4 antibody to mice bearing established MC38 syngeneic tumors. As single agents, the PD-1 antibody and Probody therapeutics reduced tumor growth relative to an isotype antibody control and synergized with CTLA-4 blockade. Consistent with their reduced ability to bind target in the absence of proteolytic activation, the PD-1 Probody therapeutics were up to 10 times less potent than the parental anti-PD-1 antibody in inducing autoimmune diabetes in NOD mice. Our preclinical findings demonstrate that PD-1 Probody therapeutics retain anti-tumor efficacy with improved safety profiles and therefore have the promise to enable better tolerated PD-1 combination immunotherapies.

PROBODY is a trademark of CytomX Therapeutics, Inc.

Citation Format: Kimberly A. Tipton, Chanty Chan, Kenneth R. Wong, Victoria Singson, Jennifer H. Richardson, W Michael Kavanaugh, Bryan A. Irving, James W. West. PD-1-targeted Probody therapeutics provide anti-tumor efficacy and a 10-fold dose protection against systemic autoimmunity in preclinical studies. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3211.

Abstract A081: A PD-L1-targeted Probody provides antitumor efficacy while minimizing induction of systemic autoimmunity

Antibodies to T cell checkpoint molecules like PD-1, PD-L1 and CTLA-4 can unleash potent and durable anti-tumor immunity in many different cancer types. These drugs induce anti-tumor immune responses by activating T cells in tumors, but because similar mechanisms control anti-tumor immunity and self-tolerance, they can also induce systemic autoimmunity by activating autoreactive T cells in normal tissues. Combinations of checkpoint inhibitors greatly increase clinical responses, but similarly increase these toxicities, thereby limiting their clinical utility. New methods are therefore needed that provide anti-tumor activity without deregulating systemic immunity.

Probody™ therapeutics are recombinant, proteolytically-activated antibody prodrugs, designed to widen therapeutic index by minimizing interaction with normal tissue and maximizing interaction with tumor. Probodies exploit the fundamental dysregulation of extracellular protease activity that exists in tumors relative to healthy tissue. The amino terminus of the antibody light chain is extended with a protease-cleavable linker and a masking peptide designed to block antigen binding to the antibody. Cleavage of the linker by specific tumor-associated proteases leads to dissociation of the mask and release of an antibody competent to bind to antigen in the tumor. Probodies bind only minimally to antigen in healthy tissue because there are insufficient active proteases present to remove the mask. The feasibility of the Probody approach to checkpoint targets is supported by preclinical studies in which intratumoral delivery of low dose immune modulators enables durable, systemic anti-tumor responses despite negligible systemic exposure.

Here we demonstrate the ability of a Probody targeting PD-L1 to provide equivalent anti-tumor efficacy in mice to that of its antibody parent, while minimizing induction of systemic autoimmunity. A mouse/human cross-reactive anti-PD-L1 antibody was generated by phage display, and engineered into a Probody. Binding to mouse PD-L1 by the Probody in its masked state was significantly reduced relative to its binding following proteolytic removal of the mask. The presence of active proteases capable of releasing the Probody's mask was demonstrated in frozen sections of MC38 and CT26 murine syngeneic tumors. Systemic administration of the PD-L1 Probody to mice bearing established MC38 syngeneic tumors induced anti-tumor efficacy comparable to the unmasked parental antibody at the same dose. In contrast, the PD-L1 Probody provided protection from induction of autoimmune diabetes in NOD mice at doses sufficient to induce maximal diabetes with the parental antibody. Finally, PD-L1 occupancy by the Probody on peripheral blood and splenic T cells was significantly reduced compared to that of the parental antibody at the same dose. This finding is consistent with the reduced ability of the Probody in its masked form to bind to PD-L1 and to induce autoimmunity outside of the tumor. By demonstrating equivalent efficacy with an improved safety profile, these results provide validation of the Probody concept for a T cell checkpoint target. Probodies should expand opportunities for immunotherapies, particularly those that are limited by enhanced toxicities when used in combination.

Citation Format: Chihunt Wong, Li Mei, Kenneth R. Wong, Elizabeth E.M. Menendez, Olga Vasiljeva, Jennifer H. Richardson, James W. West, Michael Kavanaugh, Bryan A. Irving. A PD-L1-targeted Probody provides antitumor efficacy while minimizing induction of systemic autoimmunity. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A081.

Abstract A203: CD3-EGFR bispecific Probody™ therapeutics induced tumor regressions and increased therapeutic window in preclinical studies

T cell-engaging bispecific antibodies (TCBs) represent a highly potent modality to direct the activity of cytotoxic T cells to tumors. TCBs are designed to bind both a surface tumor antigen and the CD3ϵ subunit of the T cell receptor, thereby enabling T cells to kill tumor cells. Because T cell bispecifics bypass TCR antigen recognition and co-stimulatory requirements for T cell activation, they obviate the need for tumor-specific immunity and overcome many impediments faced by T cells in the tumor-microenvironment. TCBs have shown clinical activity in hematologic malignancies as evidenced by the recent approval of blinatumomab for the treatment of relapsed refractory acute lymphoblastic leukemia based on its ability to direct T cells against CD19+ cells. Development of TCBs for non-hematologic cancers is proving more challenging, however, due to the high potency of the modality and its inability to discriminate between cells expressing target antigen in the tumor and those in healthy tissue. Because antigens with highly restricted tumor expression are rare, toxicity directed against healthy tissues has limited the therapeutic utility of TCBs for most solid tumor targets. In addition, because TCBs are administered at very low doses, preferential distribution to circulating T cells or to healthy tissues can limit drug exposure in tumors. Therefore new methods are needed that enable the potent anti-tumor activity of TCBs without on-target damage to normal tissues.

Here we describe a T cell-engaging Bispecific Probody therapeutic (TCBP) targeting CD3 and Epidermal Growth Factor Receptor (EGFR). Probody therapeutics are recombinant, proteolytically-activated antibody prodrugs designed to widen therapeutic window by minimizing interaction with normal tissues and maximizing interactions with tumors. A Probody therapeutic is an antibody in which the antigen-binding site is blocked by a masking peptide extension from the amino terminus of the light chain. The peptide mask, joined to the antibody by a protease-cleavable linker, can be removed by proteases that are more active in the tumor microenvironment than in normal tissue, allowing for tumor-localized activity. We have previously demonstrated that an EGFR-targeted Probody therapeutic limits interaction with EGFR-expressing tissue while preserving anti-tumor activity. To demonstrate that Probody technology can address the limitations of bispecific antibodies, we compared CD3-EGFR TCBPs to the corresponding unmasked TCB for their ability to direct T cell-mediated cytotoxicity of EGFR+ tumor cells in vitro. While the unmasked CD3-EGFR TCB demonstrated potent T cell- and EGFR-dependent killing at single-digit pM concentrations, the CD3-EGFR TCBP exhibited reduced targeted cytotoxicity by up to 3000-fold. In a tumor model utilizing engraftment of human T cells in NSG mice, both TCB Probody therapeutic and its parental TCB effectively eliminated established EGFR+ HT-29 colorectal tumors at equivalent doses. In contrast, an ongoing study in non-human primates has shown at least a 10-fold higher maximum tolerated dose for the CD3-EGFR TCBP relative to its corresponding CD3-EGFR TCB. As a result, Probody technology has the potential to increase therapeutic window and may enable potent bispecific therapies for solid tumors that are limited by on-target toxicities in healthy tissue.

Citation Format: Sherry L. LaPorte, Daniel R. Hostetter, Laurie Wong, Jennifer Razo, Linnea Diep, Clayton W. White, Jennifer H. Richardson, W. Michael Kavanaugh, Bryan A. Irving. CD3-EGFR bispecific Probody™ therapeutics induced tumor regressions and increased therapeutic window in preclinical studies. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A203.

The BET bromodomain inhibitor JQ1 suppresses growth of pancreatic ductal adenocarcinoma in patient-derived xenograft models

The primary aim of this study was to evaluate the antitumor efficacy of the bromodomain inhibitor JQ1 in pancreatic ductal adenocarcinoma (PDAC) patient-derived xenograft (tumorgraft) models. A secondary aim of the study was to evaluate whether JQ1 decreases expression of the oncogene c-Myc in PDAC tumors, as has been reported for other tumor types. We used five PDAC tumorgraft models that retain specific characteristics of tumors of origin to evaluate the antitumor efficacy of JQ1. Tumor-bearing mice were treated with JQ1 (50 mg/kg daily for 21 or 28 days). Expression analyses were performed with tumors harvested from host mice after treatment with JQ1 or vehicle control. An nCounter PanCancer Pathways Panel (NanoString Technologies) of 230 cancer-related genes was used to identify gene products affected by JQ1. Quantitative RT-PCR, immunohistochemistry and immunoblots were carried out to confirm that changes in RNA expression reflected changes in protein expression. JQ1 inhibited the growth of all five tumorgraft models (P<0.05), each of which harbors a KRAS mutation; but induced no consistent change in expression of c-Myc protein. Expression profiling identified CDC25B, a regulator of cell cycle progression, as one of the three RNA species (TIMP3, LMO2 and CDC25B) downregulated by JQ1 (P<0.05). Inhibition of tumor progression was more closely related to decreased expression of nuclear CDC25B than to changes in c-Myc expression. JQ1 and other agents that inhibit the function of proteins with bromodomains merit further investigation for treating PDAC tumors. Work is ongoing in our laboratory to identify effective drug combinations that include JQ1.

Tumor-specific activation of an EGFR-targeting probody enhances therapeutic index

Target-mediated toxicity constitutes a major limitation for the development of therapeutic antibodies. To redirect the activity of antibodies recognizing widely distributed targets to the site of disease, we have applied a prodrug strategy to create an epidermal growth factor receptor (EGFR)-directed Probody therapeutic-an antibody that remains masked against antigen binding until activated locally by proteases commonly active in the tumor microenvironment. In vitro, the masked Probody showed diminished antigen binding and cell-based activities, but when activated by appropriate proteases, it regained full activity compared to the parental anti-EGFR antibody cetuximab. In vivo, the Probody was largely inert in the systemic circulation of mice, but was activated within tumor tissue and showed antitumor efficacy that was similar to that of cetuximab. The Probody demonstrated markedly improved safety and increased half-life in nonhuman primates, enabling it to be dosed safely at much higher levels than cetuximab. In addition, we found that both Probody-responsive xenograft tumors and primary tumor samples from patients were capable of activating the Probody ex vivo. Probodies may therefore improve the safety profile of therapeutic antibodies without compromising efficacy of the parental antibody and may enable the wider use of empowered antibody formats such as antibody-drug conjugates and bispecifics.

Vector competence of Kenyan Culex zombaensis and Culex quinquefasciatus mosquitoes for Rift Valley fever virus

Rift Valley fever (RVF) continues to be a significant problem in Kenya as well as in Egypt, Yemen, and Saudi Arabia. In order to determine the ability of Kenyan mosquitoes to transmit RVF virus (RVFV), we collected mosquitoes in the Lake Naivasha region of Kenya and evaluated them for their potential to transmit RVFV under laboratory conditions. After feeding on a hamster (Mesocricetus auratus) with a viremia of 10(9.7) plaque-forming units of virus/ml of blood, Culex zombaensis were highly susceptible to infection with RVFV, with 89% becoming infected. In contrast, Cx. quinquefasciatus that were fed on the same hamsters were marginally susceptible, with only 20% becoming infected. Differences in percentages of mosquitoes that developed a disseminated infection were equally disparate, with 55% and 8%, for Cx. zombaensis and Cx. quinquefasciatus, respectively. Forty-eight percent of the Cx. zombaensis with a disseminated infection that fed on a susceptible hamster transmitted virus by bite, indicating a moderate salivary gland barrier. However, the presence of a salivary gland barrier could not be determined for Cx. quinquefasciatus because none of the 18 mosquitoes that took a 2nd blood meal had a disseminated infection. These studies illustrate the need to identify the ability of individual mosquito species to transmit RVFV so that correct decisions can be made concerning the application of appropriate control measures during an outbreak.

Chromosomal positioning of human T-lymphotropic type 1 proviruses by fluorescence in situ hybridisation

Fluorescence in situ hybridisation (FISH) was employed to identify the chromosomal integration site of the human T-cell lymphotropic virus, type 1 (HTLV-1) present in T-cell clones derived from HTLV-1-infected individuals and a virally transformed cell line, C8166-45. Proviral sequences were detected in C8166-45 but not uninfected Jurkat cells. Integration sites were reliably detected in T-cell clones determined previously to be infected with HTLV-1. The results indicated that the transformed cell line and some of the T-cell clones possessed more than one proviral integration site. This hybridisation system is useful for determining the number of integration events and for localising proviruses to specific chromosomal regions.

Virus inactivation in a proportion of human T-cell leukaemia virus type I-infected T-cell clones arises through naturally occurring mutations

Human T-cell leukaemia virus type I (HTLV-I) is the aetiological agent of adult T-cell leukaemia/lymphoma and tropical spastic paraparesis/HTLV-I-associated myelopathy (TSP/HAM). The trans-activating protein (Tax) of HTLV-I is strongly implicated in cellular proliferation. We examined the tax gene and 5' long terminal repeat (LTR) sequences in eight naturally infected T-cell clones derived from TSP/HAM-affected individuals who were either productively (proliferate spontaneously) or silently (do not proliferate spontaneously) infected. In two silently infected clones point mutations within the proviruses resulted in truncation of the Tax protein. One clone harboured both a deleterious tax gene mutation and a point mutation in an enhancer element of the 5' LTR. Sequence changes, immunological escape mutation, integration site context and host cell phenotype may all contribute to the high proportion of latently or silently infected T-cells found in vivo in virus carriers.

Intrabody-mediated knockout of the high-affinity IL-2 receptor in primary human T cells using a bicistronic lentivirus vector

A bicistronic human immunodeficiency virus type 1 (HIV-1)-based vector is described in which the expression of a selectable marker and a second gene of interest are forcibly coupled by means of an internal ribosome entry site. The vector provides high-level expression of the coselected gene in approximately 90% of transduced cells and has been used to express an endoplasmic reticulum-targeted single-chain antibody (intrabody) directed against a subunit of the interleukin-2 receptor, IL-2R alpha. In the established T cell line Kit225 and also in primary human T cells stably transduced with the intrabody vector, the cell surface expression of IL-2R alpha could be reduced to a low or undetectable level. Responsiveness to IL-2 was reduced 10-fold in the IL-2R alpha-negative cells, consistent with a lack of high-affinity IL-2 receptors. Pseudotyping of the HIV-1 core with the vesicular stomatitis virus G protein improved particle stability by two- to three-fold and enhanced vector entry into established T cell lines up to 230-fold. Vector entry into primary human T cells was most efficient when the amphotropic murine leukemia virus envelope was used. The forced, high-expression capability of the bicistronic vector, together with the capacity of HIV-1 vectors to infect nondividing cells, make this an attractive tool for the genetic manipulation of primary cell types.

Inducible knockout of the interleukin-2 receptor alpha chain: expression of the high-affinity IL-2 receptor is not required for the in vitro growth of HTLV-I-transformed cell lines

Adult T cell leukemia (ATL) is an aggressive malignancy that is associated with HTLV-I infection and characterized by constitutive expression of the high-affinity interleukin-2 receptor. The alpha subunit of the high-affinity receptor (IL-2Ralpha), which is normally present only on activated T cells, is specifically upregulated by HTLV-I and constitutively expressed on fresh leukemic cells from ATL patients as well as cell lines transformed by HTLV-I in vitro. Here we directly address the functional significance of IL-2Ralpha expression in HTLV-I transformed cell lines by using an endoplasmic reticulum-targeted single-chain antibody to inhibit the cell surface expression of IL-2Ralpha. Using constitutive and tetracycline-repressible systems to express the ER-targeted antibody against IL-2Ralpha, we have reduced cell surface expression of IL-2Ralpha by more that 2 logs of mean fluorescence intensity to virtually undetectable levels in the IL-2-independent HTLV-I-transformed cell lines C8166-45 and HUT102. No toxicity was associated with the intracellular retention of IL-2Ralpha, and the growth rate of the IL-2Ralpha-negative cells was in each case comparable to that of the parental cell line. We conclude that cell surface expression of IL-2Ralpha is dispensable for the in vitro growth of these HTLV-I-transformed cells.

Variable immortalizing potential and frequent virus latency in blood-derived T-cell clones infected with human T-cell leukemia virus type I

Human T-cell leukemia virus type I (HTLV-I)-infected T cells expanded in vitro by single-cell cloning provide a unique system for investigating virus-cell interactions in nonimmortalized T cells. By analysis of clones generated randomly from the blood of virus carriers, we confirm that CD4 T cells are the major reservoir of HTLV-I in vivo and show that most infected cells contain a single integrated provirus. Contrary to the situation in HTLV-I immortalized cell lines, the HTLV-I provirus was found to be transcriptionally silent in a high proportion of randomly generated T-cell clones and could not be reactivated by mitogenic stimulation. The spontaneous proliferation previously documented in HTLV-I-infected T-cell clones was not observed in silently infected cells, and therefore correlates directly with the expression of tax and other viral genes. The only cytokine mRNA found to be significantly elevated in the virus-producing clones was interleukin-6; however, receptor-blocking experiments argue against a role for IL-6 in the virus-induced cell proliferation. We observed a striking variation in the ability of individual HTLV-I-producing clones to immortalize fresh peripheral blood lymphocytes. This ability did not correlate with the levels of viral mRNA expression, gag p24 production, spontaneous proliferation, or tax-transactivation, possibly suggesting a role for host cell factors as determinants of viral infectivity or immortalization. Studies to elucidate the basis of this phenotypic heterogeneity should enhance our understanding of viral spread and pathogenesis.

Survey of container-inhabiting mosquitoes in Clemson, South Carolina, with emphasis on Aedes albopictus

A total of 530 oviposition trap samples were collected within a 10-km radius of Clemson University between March 30 and October 19, 1993. From 19,664 larvae reared from collected eggs, 7 species were identified: Aedes albopictus (89%), Ae. triseriatus (6.5%), Culex restuans (2.7%), Cx. territans (0.6%), Cx. pipiens complex (0.7%), Toxorhynchites rutilus septentrionalis (0.2%), and Orthopodomyia signifera (0.1%). This is the first record of Ae. albopictus in Clemson. Aedes aegypti was not found. Of the 41 ovitrap locations, 100% were positive for Ae. albopictus.

cis-acting sequences involved in human immunodeficiency virus type 1 RNA packaging

We have previously described a series of human immunodeficiency virus type 1-based vectors in which efficient RNA encapsidation appeared to correlate with the presence of a 1.1-kb env gene fragment encompassing the Rev-responsive element (RRE). In this report, we explore in detail the role of the RRE and flanking env sequences in vector expression and RNA encapsidation. The analysis of a new series of vectors containing deletions within the env fragment failed to identify a discrete packaging signal, although the loss of certain sequences reduced packaging efficiency three- to fourfold. Complete removal of the env fragment resulted in a 100-fold decrease in the vector transduction titer but did not abolish RNA encapsidation. We conclude that the RRE and 3' env sequences are not essential for human immunodeficiency virus type 1 vector encapsidation but may be important in vectors in which a heterologous gene has been placed adjacent to the 5' packaging signal, potentially disrupting its structure.

Intracellular antibodies: development and therapeutic potential

Single-chain antibodies, synthesized by the cell and targeted to a particular cellular compartment, can be used to interfere in a highly specific manner with cell growth and metabolism. Recent applications of this technology include the phenotypic knockout of growth-factor receptors, the functional inactivation of p21ras and the inhibition of HIV-1 replication. Intracellular antibodies are likely to have a widespread impact in biological research as a simple and effective alternative to other forms of gene inactivation; they demonstrate clear potential as reagents for cancer therapy and for the control of infectious diseases.

Phenotypic knockout of the high-affinity human interleukin 2 receptor by intracellular single-chain antibodies against the alpha subunit of the receptor

The experimental manipulation of peptide growth hormones and their cellular receptors is central to understanding the pathways governing cellular signaling and growth control. Previous work has shown that intracellular antibodies targeted to the endoplasmic reticulum (ER) can be used to capture specific proteins as they enter the ER, preventing their transport to the cell surface. Here we have used this technology to inhibit the cell surface expression of the alpha subunit of the high-affinity interleukin 2 receptor (IL-2R alpha). A single-chain variable-region fragment of the anti-Tac monoclonal antibody was constructed with a signal peptide and a C-terminal ER retention signal. Intracellular expression of the single-chain antibody was found to completely abrogate cell surface expression of IL-2R alpha in stimulated Jurkat T cells. IL-2R alpha was detectable within the Jurkat cells as an immature 40-kDa form that was sensitive to endoglycosidase H, consistent with its retention in a pre- or early Golgi compartment. A single-chain antibody lacking the ER retention signal was also able to inhibit cell surface expression of IL-2R alpha although the mechanism appeared to involve rapid degradation of the receptor chain within the ER. These intracellular antibodies will provide a valuable tool for examining the role of IL-2R alpha in T-cell activation, IL-2 signal transduction, and the deregulated growth of leukemic cells which overexpress IL-2R alpha.

Helper virus-free transfer of human immunodeficiency virus type 1 vectors

Recombinant vectors based on the type 1 human immunodeficiency virus (HIV-1) can be used to deliver genes into cells expressing the HIV receptor, CD4. We have used a transient RNA packaging system to compare the safety and efficacy of HIV-1 vector transduction by wild-type and replication-deficient helper viruses. Helper virus-free vector transfer was consistently achieved when the helper virus gag-pol and env genes were expressed from separate plasmids such that two recombination events were required to form an infectious genome. Other forms of attenuation, such as deletion of the 5' phi region, were inadequate to prevent helper virus transmission. Vector transduction by the wild-type and non-replicating helper viruses occurred with comparable efficiency except in instances where efficient vector RNA expression was dependent upon transactivating factors supplied by the helper virus. These data demonstrate the feasibility of safe gene transfer using HIV-1 vectors.

Regional specificity of HTLV-I proviral integration in the human genome

The location of HTLV-I (human T-cell leukemia virus type 1) proviral sequences in the genome of infected human cells was explored by hybridization of a viral probe with compositional fractions of host-cell DNAs. In the twelve cases examined, HTLV-I sequences were absent from the GC-poorest 40% of the host genome (namely, from isochores that are below 39% GC). Transcriptionally inactive proviral sequences were localized in GC-poor isochores (comprised between 39% and 42-44% GC) of the human genome, which are characterized by a constant and low gene concentration. In contrast, transcriptionally active proviral sequences were found in the GC-rich and very GC-rich isochores, which are gene rich, transcriptionally and recombinationally active, and endowed with an open chromatin structure. Since GC-rich isochores are present in R'-bands and very GC-rich isochores form T-bands, these results also provide information on the location of HTLV-I proviral sequences in human chromosomes. The results obtained with HTLV-I are in agreement with the non-random, compartmentalized integration of animal retroviral sequences that had been previously observed in other viral-host systems. They provide, however, much more detailed information on the regional location of proviral sequences in the host genome and on the correlation between their transcription and their location.

Packaging of human immunodeficiency virus type 1 RNA requires cis-acting sequences outside the 5' leader region

cis elements required for the encapsidation of human immunodeficiency virus type 1 (HIV-1) RNA have been investigated by using a replication-competent helper virus to package a series of HIV-1-based vectors which had been stably transfected into human CD4 T-cell lines. A previously identified packaging signal in the 5' leader region was not sufficient for the encapsidation of small vectors containing heterologous genes. In contrast, vectors containing additional gag and env sequences were packaged with high efficiency and transduced into CD4-expressing target cells with titers exceeding 10(4) CFU/ml. The presence of gag sequences did not enhance vector packaging efficiency. A 1.1-kb env gene fragment encompassing the Rev-responsive element was absolutely required for the expression and encapsidation of vectors containing cis-acting repressive sequences and appeared also to contain an important packaging signal. Vectors as small as 2.6 kb were successfully packaged in this system. The presence of abundant, packageable vector RNA did not appear to interfere with encapsidation of the wild-type HIV-1 genome, suggesting that HIV-1 RNA packaging capacity is not saturated during acute infection.

T-cell activation by autologous human T-cell leukemia virus type I-infected T-cell clones

A unique feature of both human T-cell leukemia virus type I (HTLV-I) carriers and subjects with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a chronic inflammatory disease of the nervous system, is the presence of large numbers of activated T cells that spontaneously proliferate in vitro. We have investigated the mechanisms of T-cell activation by HTLV-I in freshly isolated blood T cells and in naturally infected T-cell clones obtained by direct single-cell cloning from patients with HAM/TSP. Both CD4+ and CD8+ HTLV-I-infected T-cell clones showed the unusual ability to proliferate in the absence of exogenous interleukin 2 (IL-2). Nevertheless, HTLV-I-infected clones were not transformed, as they required periodic restimulation with phytohemagglutinin and feeder cells for long-term growth. Irradiated or fixed HTLV-I-infected clones were found to induce the proliferation of blood T cells when cocultured, which we refer to as THTLV-1-T cell activation. This THTLV-1-T cell-mediated activation was blocked by monoclonal antibodies (mAbs) against CD2/lymphocyte function-associated molecule 3 (LFA-3), LFA-1/intercellular cell-adhesion molecule (ICAM), and the IL-2 receptor but not by mAbs against class I or class II major histocompatibility complex molecules, HTLV-I gp46, or a high-titer HAM/TSP serum. Spontaneous proliferation of blood T cells from HAM/TSP patients could also be inhibited by mAbs to CD2/LFA-3, LFA-1/ICAM and to the IL-2 receptor (CD25). These results show at the clonal level that HTLV-I infection induces T-cell activation and that such activated T cells can in turn stimulate noninfected T cells by cognate THTLV-1-T cell interactions involving the CD2 pathway.

Biological safety cabinetry

The biological safety cabinet is the one piece of laboratory and pharmacy equipment that provides protection for personnel, the product, and the environment. Through the history of laboratory-acquired infections from the earliest published case to the emergence of hepatitis B and AIDS, the need for health care worker protection is described. A brief description with design, construction, function, and production capabilities is provided for class I and class III safety cabinets. The development of the high-efficiency particulate air filter provided the impetus for clean room technology, from which evolved the class II laminar flow biological safety cabinet. The clean room concept was advanced when the horizontal airflow clean bench was manufactured; it became popular in pharmacies for preparing intravenous solutions because the product was protected. However, as with infectious microorganisms and laboratory workers, individual sensitization to antibiotics and the advent of hazardous antineoplastic agents changed the thinking of pharmacists and nurses, and they began to use the class II safety cabinet to prevent adverse personnel reactions to the drugs. How the class II safety cabinet became the mainstay in laboratories and pharmacies is described, and insight is provided into the formulation of National Sanitation Foundation standard number 49 and its revisions. The working operations of a class II cabinet are described, as are the variations of the four types with regard to design, function, air velocity profiles, and the use of toxins. The main certification procedures are explained, with examples of improper or incorrect certifications. The required levels of containment for microorganisms are given. Instructions for decontaminating the class II biological safety cabinet of infectious agents are provided; unfortunately, there is no method for decontaminating the cabinet of antineoplastic agents.

Screening for prolonged incubation of HTLV-I infection in British and Jamaican relatives of British patients with tropical spastic paraparesis

OBJECTIVE

To compare the prevalence of antibody to and proviral DNA of the retrovirus HTLV-I in relatives of 11 British patients with tropical spastic paraparesis who had migrated from Jamaica before they developed symptoms, and to examine factors possibly related to transmission of HTLV-I.

DESIGN

Migrant, family study. Antibody state was determined by several methods and confirmed by western blotting; the polymerase chain reaction was used to detect proviral DNA.

SETTING

Britain and Jamaica.

SUBJECTS

All available first degree relatives: those born and still resident in Jamaica (group 1); those born in Jamaica who migrated to Britain (group 2); and index patients' children who were born and resident in Britain (group 3). All had been breast fed and none had had blood transfusions.

RESULTS

Of the 66 living relatives, 60 were traced. Seroprevalence among those born in Jamaica (irrespective of current residence) was 22% (10/46; 95% confidence limits 9 to 34%) compared with zero among British born offspring (0/14) and was higher in group 2 at 33% (7/21; 12 to 55%) than in group 1 at 12% (3/25; 0 to 25%). (Patients in group 1 had the greatest mean age.) Proviral DNA was not detected in any subject negative for HTLV-I antibody, making prolonged viral incubation in those negative for the antibody unlikely.

CONCLUSION

In this sample factors related to place of birth and early residence were more important in transmission of HTLV-I than maternal or age effects. In areas with a low to moderate prevalence policies of preventing mothers who are carriers of the virus from breast feeding would be premature.

Effect of vitamin E on vitamin C-induced lymphocytopenia

Large daily doses of vitamin C resulted in lymphocytopenia in stressed mice caused by excessive plasma levels of adrenal corticosteroids. Vitamin C plus E resulted in less severe lymphocytopenia because vitamin E has a lymphoproliferative effect.

Aggressive behavior and lymphocyte damage

Aggressive and passive mice were placed together in one cage. Lymphocyte counts were taken for both groups over a 10-day period. Although lymphocyte damage occurred, there was no significant difference in the severity of lymphocyte damage between the two groups.

Effect of ascorbic acid on pituitary weight during stress

40 Sprague-Dawley rats were subjected to stress by crowding and water immersion for five days. Three groups of animals were compared: controls ( n = 20), stressed animals dosed with ascorbic acid ( n = 20), and stressed animals not dosed with ascorbic acid ( n = 20). Ascorbic acid prevented both adrenal and pituitary hypertrophy in stressed animals. Ascorbic acid appears to act as a potentiating factor for ACTH when there is an increased demand for ACTH during stress.

Dietary supplementation with vitamin C delays the onset of fatigue in isolated striated muscle of rats

The purpose of this study was to assay the effect of prolonged vitamin C supplementation on contraction time and strength in the gastrocnemius muscle of the rat. Fifteen male Sprague-Dawley rats were given 30 mg of vitamin C orally per day for thirty days, while an additional fifteen animals served as controls. Contraction of the isolated gastrocnemius muscle was induced by electrical stimulation, and strength and time to fatigue was measured. Results indicate that the supplementation of vitamin C prolongs contraction time by 19% thus delaying fatigue but had no affect on muscle strength.

Acquired megacolon associated with acute lead poisoning

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Comparison of accuracy in five measures of stress

36 Sprague-Dawley rats were subjected to stress by immersion, erratic photoperiod and crowding for 28 days. Control and experimental animals were sacrificed and the thymus, spleen, adrenals, testis, and pituitary were removed and their weights were recorded as a ratio of organ weight to body weight. The pituitary was the most accurate measure of stress according to Z scores.