Infectious Disorders - Drug Targets (Formerly Current Drug Targets - Infectious Disorders) - Volume 8, Issue 4, 2008

In spite the great advances on biomedical sciences, and in particular on the analysis of the immune response regarding cells, mechanisms, signaling pathways and genes involved, together with a plethora of genetically modified animal models, infectious diseases still keep increasing in the world population, both on terms of morbidity and mortality. Naive CD4⁺ T lymphocytes, upon encountering antigen, differentiate towards Th1 or Th2 cells, and the signals leading to the induction of one or the other T cell subset are well known. Similarly, the mechanisms used by CD8⁺cytotoxic T cells to kill their specific targets have been unraveled and conditions leading to the induction of different phenotypes (Tc1, Tc2) established. However, this increased knowledge on immune response mechanisms (both cellular and molecular) has not yet been reflected on the identification of new drug targets to fight infection. The aim of this special issue is to discuss the reasons that might explain, at least in part and form the point of view of the authors, the lack of new drug targets on infectious diseases. The first manuscript by R. Vernal & J.A. Garcia-Sanz discusses two new T lymphocyte subpopulations, namely Th17 cells and regulatory T cells, with a profound impact on the outcome of the T cell responses, analyzing in particular their role in different infections. A manuscript by F. Erard & B. Ryffel follows in which the potential of Toll like receptors as potential drug targets on infectious diseases is discussed. Toll like receptors play a crucial role in the recognition and response to pathogens by the innate immune system. The use of a particular set of TLR could favor a Th1-biased response versus a Th2-biased response, or vice-versa. The following group of manuscripts review several mechanisms that have either been underscored or neglected in the past, all of them with a high impact on T cell activation and acquisition of effector functions. The first is by J. Nakagawa on mRNA stability, where the author describes the relevance of the process, and the current knowledge on the mechanisms involved. The second by O. Villate et al., describes the new increased complexity of the mammalian genomes due to the extensive usage of alternative splicing to generate different isoforms and their role in several diseases. The third one by E. Diaz-Guerra et al., discuss the issue of translational control as a mechanism regulating gene expression and describe evidences not only of the effects of different infections on the translation of the host cell, but also on the own infectious agent. Finally, as a possible application of some of these tools to drug discovery, M. Lopez-Fraga et al., describe the mechanisms of RNA interference and the possible use of siRNA as a new strategy to identify new drug targets.

In addition to the T helper 1 (Th1) and Th2 lymphocyte subsets, two new subpopulations Th17 and regulatory T (Treg) cells have recently been described. Th17 cells, which produce high levels of interleukin (IL)-17, are dependent on the transcription factor orphan nuclear receptor RORC2/RORγt and have been implicated in exacerbating the immune response to infections. Conversely, Treg cells, either thymus-derived or generated upon TCR activation of naïve T cells, express the transcription factor forkhead box P3 (Foxp3) and have regulatory functions mediated through either direct cell-cell contact or immuno-suppressive cytokines, being able to suppress the activation of T, B and NK cells. Based on the current knowledge of Th17 and Treg cell functions, new therapeutic strategies start to emerge, involving anti-cytokine treatments targeting Th17 functions or cell-based treatments in which Treg cells are generated from T cells either through Foxp3 gene transfer onto T cells with known specificities or transferring specific TCR genes onto Treg cells.

Toll like receptors (TLR) play a critical role in the recognition and response of pathogens by the innate immune system. Pathogen engagement of the TLR-MyD88 pathway favours the development of a protective Th1-biased T cell response. Interruption of TLR recognition or signalling has profound effects on innate immunity. Agonists or antagonists of specific TLRs modulate the host response to microbial infections and have effects beyond infectious control and may be used as immunostimulators in vaccine, cancer, inflammatory disorders and allergy.

Posttranscriptional regulation of gene expression plays a pivotal role as a fast control system for T-cells and Bcells operating in the defense reactions against rapidly growing infectious agents. The framework of this machinery involves cis-acting elements in the mRNAs of relevant cytokines and trans-acting factors interacting with these elements. The cis- and trans-acting factors enforce rapid mRNA decay with other proteins such as nucleases in the decay machinery. The most prominent cis-element contains A + U- rich sequence (ARE), and is located in the 3'-untranslated region of the target mRNAs. Some ARE-binding proteins promote the rapid decay, and others protect the mRNA from degradation. The 5'-end of nascent mRNA undergoes capping which protects the 5'-end together with the cap-binding protein, and the 3' end is protected with poly (A) tail and associating poly (A) binding protein. Unlike in classical drawing of linear structure of mRNA, the end structures interact with each other through a common platform composed of translation initiation factors, revealing the cross-talk of the 5'-end cap structure and 3'-end poly (A) tail on the translational machinery. The rapid degradation and stabilization of mRNA is triggered by a cellular signaling cascade through phosphorylation of associating protein factors in response to environmental stimuli, and a large nucleolytic complex for specific decay reaction called exosome is formed with the 3'-UTR of mRNA through interaction with the ARE-binding proteins. Possible therapeutic agents modifying stability of ARE-containing mRNA are being screened in order to treat immunological disorders.

Genome complexity and diversity can be due to Alternative Splicing (AS), a process by which one gene can generate multiple mRNA isoforms and then several proteins. This is part of a normal process of variation on an individual, and when it is disrupted or modified, may trigger disease. To date, there are many pathologies described due to the effects of altered splicing isoforms, and effort is focused on the description of new ones. The design of drug target has to consider splicing, as in many occasions, a drug might have effect on different isoforms, instead of on the particular one implicated in the pathology. Interestingly, the strategies used to alter splicing can be used to modify a form towards the canonical one, or towards an aberrant one, when the latter one has a beneficial effect on the individual. Here we describe differential splicing, diseases produced by alterations on the mRNA isoforms, and drugs or methods used to restore these alterations.

Recent data from a series of laboratories has pinpointed the relevant role of translation control on the regulation of gene expression. In particular, an analysis of T cell activation has led to demonstrate that during this physiological transition about 20% of the regulated mRNAs are controlled at the translation level. Furthermore, modulating the host mRNA translation is one of the mechanisms used by infectious agents to achieve a productive infection. In the present review, we summarize the current knowledge on the translation machinery, the translational control mechanisms during the immune response, as well as the mechanisms used by different pathogens to avoid, inhibit or postpone the host response; and suggest that the analysis on genome-wide screening of the host-pathogen interactions, identifying translationally regulated mRNAs, might unravel new drug targets in infectious diseases.

For many years, there has been an ongoing search for new compounds that can selectively alter gene expression as a new way to treat human disease by addressing targets that are otherwise “undruggable” with traditional pharmaceutical approaches involving small molecules or proteins. RNA interference (RNAi) strategies have raised a lot of attention and several compounds are currently being tested in clinical trials. Viruses are the obvious target for RNAitherapy, as most are difficult to treat with conventional drugs, they become rapidly resistant to drug treatment and their genes differ substantially from human genes, minimizing side effects. Antisense strategy offers very high target specificity, i.e., any viral sequence could potentially be targeted using the complementary oligonucleotide sequence. Consequently, new antisense-based therapeutics have the potential to lead a revolution in the anti-infective drug development field. Additionally, the relatively short turnaround for efficacy testing of potential RNAi molecules and that any pathogen is theoretically amenable to rapid targeting, make them invaluable tools for treating a wide range of diseases. This review will focus on some of the current efforts to treat infectious disease with RNAi-based therapies and some of the obstacles that have appeared on the road to successful clinical intervention.