Identificados novos anticorpos do vírus da SIDA

Descoberta abre novas pistas para investigação da vacina contra a doença

Detalhes da estrutura de partículas do vírus HIV Cientistas do Instituto de Investigação Scripps, nos Estados Unidos da América, identificaram novos e poderosos anticorpos do vírus da SIDA, que abrem novas pistas para a investigação da vacina contra a doença, avança a LUSA. Os anticorpos são 'armas' de defesa do sistema imunitário contra os agentes infecciosos, vírus ou bactérias. No caso do VIH, devido às múltiplas e rápidas mutações, os anticorpos produzidos pelos seropositivos são incapazes, na maioria das situações, de neutralizar o vírus. De acordo com o estudo publicado na revista científica britânica Nature, os 17 novos anticorpos identificados foram isolados depois de extraídos de quatro seropositivos com uma forte resposta imunitária contra o vírus da SIDA. A maior parte destes anticorpos é dez vezes mais poderosa do que os descritos recentemente (PG9, PG16 e VRC01) no âmbito das investigações sobre a vacina contra a SIDA, salientou a equipa de cientistas norte-americana. Os investigadores realçaram que certas combinações destes anticorpos conseguem um nível favorável de cobertura contra uma grande proporção de variantes do VIH. Segundo os médicos, uma vacina capaz de impedir uma infecção com o vírus da SIDA deverá desencadear no sistema imunitário a produção deste tipo de anticorpos antes de uma exposição ao agente patogénico. 

2011-08-18

Fonte:  http://www.cienciahoje.pt/index.php?oid=50520&op=all

Original: http://www.scripps.edu/ 

Differential microRNA regulation of HLA-C expression and its association with HIV control

The HLA-C locus is distinct relative to the other classical HLA class I loci in that it has relatively limited polymorphism1, lower expression on the cell surface2, 3, and more extensive ligand–receptor interactions with killer-cell immunoglobulin-like receptors4. A single nucleotide polymorphism (SNP) 35 kb upstream of HLA-C (rs9264942; termed −35) associates with control of HIV5, 6, 7, and with levels of HLA-C messenger RNA transcripts8 and cell-surface expression7, but the mechanism underlying its varied expression is unknown. We proposed that the −35 SNP is not the causal variant for differential HLA-C expression, but rather is marking another polymorphism that directly affects levels of HLA-C7. Here we show that variation within the 3′ untranslated region (UTR) of HLA-C regulates binding of the microRNA hsa-miR-148 to its target site, resulting in relatively low surface expression of alleles that bind this microRNA and high expression of HLA-C alleles that escape post-transcriptional regulation. The 3′ UTR variant associates strongly with control of HIV, potentially adding to the effects of genetic variation encoding the peptide-binding region of the HLA class I loci. Variation in HLA-C expression adds another layer of diversity to this highly polymorphic locus that must be considered when deciphering the function of these molecules in health and disease.

MicroRNAs (miRNAs) are a class of non-protein-coding RNAs that are estimated to regulate 30% of all genes in animals9 by binding to specific sites in the 3′ UTR, resulting in post-transcriptional repression, cleavage or destabilization10, 11, 12. The 3′ UTR of the HLA-C gene is predicted to be a target for 26 distinct human miRNAs using three miRNA-target-prediction programs (Supplementary Fig. 1), of which three (miR-148a and miR-148b, which bind the same target site, and miR-657) were shown to have the greatest likelihood of binding. We sequenced the 3′ UTRs of the common HLA-C alleles (Supplementary Fig. 2) and show that the two binding sites of these three miRNAs are polymorphic (Supplementary Fig. 3a). The binding site for miR-148a/miR-148b contains a single base pair insertion/deletion at position 263 downstream of the HLA-C stop codon (rs67384697G representing the insertion (263ins) and rs67384697− representing the deletion (263del)) along with other precisely linked variants (259C/T, 261T/C, 266C/T). These variants are likely to impose a restriction in miR-148a/miR-148b binding, as prediction algorithms indicate that the binding of these miRNAs to the alleles marked by 263ins (for example, Cw*0702, a low-expression allotype) is more stable than to alleles with 263del (for example, Cw*0602, a high-expression allotype) (Supplementary Fig. 3b). Similarly, alleles with 307C within the miR-657 target site are predicted to be better targets of miR-657 than those with 307T (Supplementary Fig. 4). Thus, variation in the 3′ UTR of HLA-C may influence the interaction between these miRNAs and their putative binding sites in an allele-specific manner, potentially leading to differential levels of HLA-C allotype expression.

To test directly whether the variation in the HLA-C 3′ UTR affects levels of protein expression, the full-length 3′ UTRs containing intact miR-148a/miR-148b- and miR-657-binding sites (that is, 263ins and 307C, respectively; Cw*0702, Cw*0303, Cw*0401, Cw*0701) and disrupted binding sites (that is, 263del and 307T, respectively; Cw*0602, Cw*0802, Cw*1203, Cw*1502) were each cloned downstream of the luciferase gene in a pGL3 reporter construct (Fig. 1a). The constructs were then transfected into HLA class I negative B721.221 cells, and the level of luciferase activity was measured (fold increase of relative light units). Although the Cw*0602 3′ UTR repressed luciferase activity as compared to the control containing no 3′ UTR, the constructs containing intact miRNA-binding sites (that is, 263ins and 307C; Cw*0702, Cw*0303, Cw*0401, Cw*0701) produced significantly lower luciferase activity relative to the construct containing the 3′ UTR of Cw*0602, which contains 263del and 307T (Fig. 1b). However, 3′ UTRs from other alleles with the 263del and 307T variants (Cw*0802, Cw*1203, Cw*1502) did not show significant variation in luciferase activity as compared to Cw*0602 (Fig. 1b). Psicheck2 reporter constructs containing 3′ UTRs of Cw*0602 also produced significantly higher luciferase activity as compared to those with Cw*0702 3′ UTR (Supplementary Fig. 5a), indicating that this effect was reproducible in a distinct reporter construct. Further, pGL3 constructs containing 3′ UTRs of Cw*0602 and Cw*0702 in three additional cell lines showed the same pattern as that seen in B721.221 cells, indicating a consistent difference of these 3′ UTRs in the regulation of HLA-C expression that is independent of cell type (Supplementary Fig. 5b–e). Thus, HLA-C 3′ UTR alleles characterized by variation at positions 263 and 307 within miRNA-binding regions differentially regulate gene expression.

Smita Kulkarni, Ram Savan, Ying Qi, Xiaojiang Gao, Yuko Yuki, Sara E. Bass, Maureen P. Martin, Peter Hunt, Steven G. Deeks, Amalio Telenti, Florencia Pereyra, David Goldstein, Steven Wolinsky, Bruce Walker, Howard A. Young & Mary Carrington

Full article: http://www.nature.com/nature/journal/v472/n7344/full/nature09914.html

HIV infection theory challenged

T cells are lost at a slow rate

A longstanding theory of how HIV slowly depletes the body's capacity to fight infection is wrong, scientists say.

HIV attacks human immune cells, called T helper cells. Loss of these cells is gradual, often taking many years.

It was thought infected cells produced more HIV particles and that this caused the body to activate more T cells which in turn were infected and killed.

Modelling by UK and US researchers suggests that, if that was true, cells would die out in months not years.

The study, led by Emory University in Atlanta and the Institute of Child Health in London, was published in the journal PLoS Medicine.

If the specific process by which HIV depletes this kind of white blood cell can be identified, it could pave the way for potential new approaches to treatment

Professor Jaroslav Stark

The researchers used a mathematical model of the processes by which T cells are produced and eliminated.

Using this they showed that the current theory of an uncontrolled cycle of T cell activation, infection, HIV production and cell destruction - dubbed the "runaway" hypothesis - was flawed.

They concluded that it could not explain the very slow pace of depletion that occurs in HIV infection.

If the theory were correct, then T helper cell numbers would fall to very low levels over a number of months, not years

Lack of certainty

Researcher Professor Jaroslav Stark, from Imperial College London, said: "Scientists have never had a full understanding of the processes by which T helper cells are depleted in HIV, and therefore they've been unable to fully explain why HIV destroys the body's supply of these cells at such a slow rate.

"Our new interdisciplinary research has thrown serious doubt on one popular theory of how HIV affects these cells, and means that further studies are required to understand the mechanism behind HIV's distinctive slow process of cellular destruction."

The researchers think one possible explanation could be that the virus slowly adapts itself over the course of the infection.

But they stress that further analysis is needed to verify this alternative theory.

Professor Stark said: "If the specific process by which HIV depletes this kind of white blood cell can be identified, it could pave the way for potential new approaches to treatment."

Roger Pebody, a treatment advisor at HIV charity Terrence Higgins Trust, said: "HIV is an incredibly complex virus and research is ongoing to try and establish exactly how it works.

"We need more studies in this area before we can draw any clear conclusions."

Fonte: http://news.bbc.co.uk/2/hi/6230580.stm