Coreceptors and their role in HIV infection

HIV could infect a human in two different ways. Trans-infection means the infection of CD4⁺ T cells via infectious synapse with dendritic cells (DC) where different receptors (e.g. DC-SIGN) or exosomes are involved. The other possibility is called cis-infection and leads to the direct infection of DC’s. Infected DC’s could act as a viral reservoir in vivo [1].

The infection of a cell with HIV needs the receptor CD4 and a coreceptor. CCR5 and CXCR4 play the crucial coreceptors in vivo even though other coreceptors were defined in vitro. CD4 is mainly expressed on T cells, monocytes, macrophages and subtypes of DC’s [2].

In the early phase of an HIV infection are mainly CCR5 using HIV called R5 virus detectable whereas in the late phase of the infection a shift to CXCR4 using HIV (called X4 virus) can be observed. It also known that dual-tropic viruses exists which can use CCR5 as well as CXCR4 as coreceptor.

After the attachment of HIV to a target cell via gp120:CD4 interaction the V3 loop together with the β-19 strand and the bridging sheet of the gp120 protein will bind to the coreceptor following a conformational change. After the binding of the V3 loop the gp41 protein of HIV will penetrate the cell membrane and the virus will fusion with the cell and introduce the capsid into the cell [2,3].

The use of CCR5 or CXCR4 as coreceptor depends on the amino acid sequence and the charge of the V3 loop. The V3 loop and both coreceptors are charged on the basis of acidic and basic amino acids as well as post-transcriptionally modifications. From this follows that electrostatic interactions are mainly involved within the binding of the V3 loop to a coreceptor. It is known that R5 viruses have a lower net charge in the V3 loop than X4 variants [2].

There are different prediction tools to identify the coreceptor usage. An early method is the 11/25 rule which predict the usage on the basis of basic amino acids on the positions 11 and 25 within the V3 loop. Other tools use more complex statistical models. The most popular tools are WetCat, WebPSSM and geno2pheno_[coreceptor]. The geno2pheno_[coreceptor] is based on the statistical model SVM (support vector matrices) whereas WebPSSM use position-specific scoring matrices (PSSM). WetCat use decision trees, SVM and the charge rule 11/25 [2]. The prediction of coreceptor usage plays an important role within the retroviral therapy called HAART cause of the new drug class called entry inhibitors but also within the determination of the infection status [2]. The new drug class will be discussed in further blog articles on this page.

  

 

 

 

 

 

[1]           Dong, C.; Janas, M. A.; Wang, J-H.; Olson, W. J. and Wu, L. Characterization of human immunodeficiency virus type 1 replication in immature and mature dendritic cells reveals dissociable cis- and trans-Infection. Journal of Virology 81, 11352-11362 (2007)

 

[2]           Jäger, H. Entry Inhibitoren. Neue Formen der HIV-Therapie. ISBN 978-3-540-78357-2. Springer (2008 )

[3]           Levy, A. J. HIV and the Pathogenesis of AIDS, 3th edition. ISBN 978-1-555-81393-2. ASM Press Washington DC (2007)

Regulatory T cells – between benefical and damaging effects

Preventing the body from autoimmune disease and chronic disease as well as from a too weak immune response is a crucial function of the immune regulation. Different mechanisms of immune regulation are known. One of the most important part of the peripheral immune regulation, the so-called peripheral tolerance, are the regulatory T cells, also known as Treg. On the one hand Treg has a important role in preventing  autoimmune disease like type 1 diabetes and others. On the other hand they obstruct the response against tumours and certain pathogens.

Regulatory T cells are characterized by FOXP3 activity. FOXP3 is required for the maintenance, function and development of regulatory T cells. FOXP3 is not the only factor for these scopes but the absence of FOXP3 leads to severe autoimmune disease. It is also known that thymus-derived Treg naturally express CD25 and CD4. This is the main group of regulatory T cells.

Suppression mechanisms can be divided in four parts and is possible through inhibitory cytokines, cytolysis, metabolic disruption and modulation of DC maturation and function.

There are several inhibitory cytokines which modulate Treg and leads to the suppression of effector T cells. It is known that IL-10 and also TGFβ play an important role within the Regulation of the immune response by Treg. If these cytokines are essential for the Treg-derived modulation of the immunity remains controversial. Unlike in vitro studies in vivo studies has shown that a decreased concentration of IL-10 could lead to increased inflammation and hyper-reactivity during allergic diseases. IL10 interacts with and down regulates the expression of NF-κβ. This leads to a decreased differentiation of Th1, Th2 and CD8+ cells. It is important to mention that the depletion of Treg-derived IL-10 do not lead to autoimmunity but could enhance the pathology in the colon and the lung. There is also evidence that the microenviroments of a tumour leads to a increased release of IL-10 by Treg. This results in a increased suppression of the immune response against cancer.

The second type of suppression is caused by cytolysis. Activated regulatory T cells express granzyme A and the Treg-mediated elimination of the target is caused by granzyme A and Perforin through the adhesion of CD18. It also known that Treg express granzyme B. If Treg fails to produce granzyme B the suppression activity is decreased. Treg also suppress the immune response against tumours while Treg kills NK cells and CTLs in a granzyme B- and perforin-manner. There are other apoptosis-inducing ways to suppress the immune system by Treg. One example is the TRAIL-DR5 or the galectin-1 pathway.

A further way to suppress the immune system by Treg is by metabolic disruption of the effector T-cell target. It remains to definitely determine if Treg could grab IL-2 through the high expression of CD25. This would result in a local depletion of IL-2. Dividing effector T cells rely to IL-2 and undergoes apoptosis in the absence of IL-2. But this phenomenon is not the only requirement for Treg to suppress effector T cells. There are further mechanisms like the release of adenosine nucleosides from Treg. These nucleosides activate the adenosine receptor A2AR and suppress the effector T cells. Simultaneous the binding of adenosine nucleosides to A2AR enhance the formation of Treg by inhibiting the expression of IL-6. The formation will be supported by the enhanced secretion of TGFβ. A third metabolic disruption mechanism is the direct transfer of cAMP from Treg to effector T cells via gap junctions.

The fourth mechanism of suppression is the impact of dendritic cells (DC). It is proposed that the expression of cytotoxic T-lymphocyte antigen 4 (CTLA4) by Treg leads to a reduced reactivity of DC when CTLA4 interacts with CD80 and/or CD86 expressed on DC. A further molecule which suppress effector T cell is the 2,3-dioxygenase. This molecule will be secreted when Treg interfere with DC and leads to the production of pro-apoptotic factors. Many other suppressing mechanisms via dendritic cells are known.

It is also important to mention that it will be suggested that Treg need cell-cell-contact for their suppression ability. Nevertheless it should be possible for Treg to supress effector cells just in proximity with high concetrations of supression mediators such as IL-2. Currently it is not known how much the regulation of the immune response by Treg is via antigen presenting cells (APC) such as DC or directly via effector T cells. In vivo studies have shown that Treg are more found in proximity of DC than effector T cells. More investigations is needed to understand the primary target of Treg and how the regulation by Treg happened.

“HIV-Lüge” – wissenschaftliche Fakten anstatt Weltverbesser-Beiträge

In etlichen Blogeinträgen wird die Infektion von HIV (human immunodeficiency virus), sogar die Existenz des Viruses verleugnet und somit die Schwere dieser Infektionskrankheit herunter gespielt. Manche Beiträge argumentieren zwar mit wissenschaftlichen Fakten, jedoch sind die Interprationen dieser Fakten teilweise falsch und ohne Begründung, ja teilweise sogar lächerlich.

Einige argumentieren, dass die Reverse Transkriptase (RT) in nicht mit HIV infizierten humanen Zellen gefunden wurde. Es ist jedoch nicht angegeben, ob es sich um Enzymaktivität oder Gensequenzen handelt. Es sei hier gesagt (Wie schon in einem frühren Beitrag meinerseits), dass bis zu ca. 7 %  des menschlichen Genoms aus sogenannten endogenen retroviralren Sequenzen besteht und auf frühere Infektion von Retroviren und die Intregration von viraler DNA in das humane Genom hinweisen.

Jede Zelle besitzt im Nucleus selber eine Reverse Transcriptase die sogenannte human telomerase reverse transcriptase (hTERT). Jedoch ist diese dafür verantwortlich die Telomere, die sich bei der Replikation des Genoms verkürzen, wieder zu verlängern. Die hTERT ist spezifisch für eine kleine sich wiederholende Sequenz (5′-TTAGGG-3′) und die virale Sequenz von HIV besteht, wenn wundert es, nicht aus mehreren hundert Wiederholungen der Sequenz TTAGGG. Wer es nicht glaubt darf gerne auf Pubmed.com nachsehen. Ein weiteres Problem besteht darin, dass die virale RT kein Proof-reading system besitzt und somit eine hohe Fehlerrate beim Einbau von Nukleotiden aufweist. Jedoch bleibt die Telomer-Sequenz immer TTAGGG…

Ein weiteres Indiz dafür, dass HIV existiert, ist die genetische Diversität, die zwischen denn einzelnen Gruppen (M, N, O) bis zu 35% beträgt. Woher kommen die isolierten DNA-Sequenzen mit dieser hohen genetischen Diversität? Da die genetische Diversität zwischen Menschen und Schimpansen nur ca. 1.2 – 2% beträgt, erscheint es mir ehrlich gesagt unwahrscheinlich, dass in den Menschen Sequenzen von gleichen Genen vorkommen, die sich bis 35% unterscheiden.

Eine weitere Geschichte ist die Isolierung von AIDS-Viren. Es gibt keine AIDS Viren. AIDS (Aquired immunodeficiency syndrome) ist ein Syndrom, dass heisst ein Krankheitsbild. AIDS wird diagnostiziert, wenn die CD4+ T -Zellzahl unter 200 pro Mikroliter Blut ausfällt und HI-Viren durch ELISA, Immunoblot oder RT-PCR nachgewiesen werden können. Jedoch ist die Diagnostik von HIV eine andere sehr intereassante und gleichzeitig komplizierte Thematik. Also weiterhin viel Spass nach der Suche von AIDS-Viren.

Warum lassen sich den HI-Viren nicht so einfach isolieren? Durch sehr sensitive Methoden, wie die RT-PCR kann virale RNA während einer Infektion nachgewiesen werden. Die Anzahl der viralen RNA und der infektiösen Partikel im Blut hängt jedoch sehr stark mit dem Immunsystem zusammen. Man unterscheidet dabei zwischen long-term non-progressor, rapid progressor und normal progressor. Dabei sind genetische Umstände, sogenannte Host genetic susceptibility (e.g. MHC-II Varianten und CCR5 Mutationen), genetische Diversität von HIV und weitere Einflüsse (e.g. Coinfektionen mit HSV oder Antikörper, die den Eintritt von HIV in die Zellen durch die Bindung vom Antikörper an den FcR der Zelle fördern kann) verantwortlich.

Die Isolierung von infektiösen Partikeln erweist sich als schwer, da während der latenten Phase oder bei HAART Therapie der sogenannte viral load auf ein nicht detektierbares Level reduziert wird. Es kann sogar sein, dass virale RNA, die in einer um ein vielfaches höhren Anzahl im Blut vorhanden ist, nicht mehr detektierbar ist. Jedoch bleibt die Isolierung von proviraler DNA immer gleich, da die im menschlichen Genom eingebaute virale DNA nicht beeinflusst wird.

Zusätzlich ist die Infektiösität von freien HI-Viren kleiner, als an Zellen (e.g. DC-SIGN von DC) angeheftete Viren.

HIV ist eine komplexe Infektionskrankheit, die nicht nur mit virologischen Erklärungen definiert werden kann. Die Problematik ist weitgehend umfassender und es müssen weitere Bereiche mit einbezogen werden, so zum Beispiel Genetik, Immunolgie, Epidemiologie, klinische Erfahrungen etc. HIV nur aus einem Blickwinkel anzusehen beruht auf einem veralteten Prinzip Infektionskrankheiten zu definieren und die koch’schen Postulate sind nicht auf jede Infektionskrankheit anzuwenden, da sich nicht jede Infektionskrankheit nach den einfachen Verhaltensmuster von bakteriellen Infektionen verhält. Ein weiteres Beispiel dafür sind Prionen. 

Die Existenz von HIV zu verleugnen, beruht nach meinen Ansichten darauf, dass die unbequeme Thematik umgangen wird und aus Bequemlichkeit, sich der Thematik nicht in einer gesamthaften Betrachtung zu widmen. Jedoch führt diese Art und Weise , neben dem gefährlichen Effekt, dass diese STD nicht mehr ernst genommen werden könnte, nicht zum erwünschten Ziel, HIV vollumfänglich zu begreifen, auch wenn eine Heilung vielleicht nie möglich sein wird.

 

Literatur:

Levy A. J. HIV and the Pathogenesis of AIDS, 3th edition. ASM Press Washington DC (2007)

Nature Reviews Immunology

Nature Reviews Molecular Cell Biology

The humoral immunity – B cells take center stage

Antibodies are important in the immunity against extracellular pathogens, but also against intracellular parasites, which spread through the extracellular space. They can  support the immunity in three ways. Antibodies neutralize a pathogen which rely to intracellular metabolism or toxins and embarass them to enter the cell through the cell membrane via receptors.

Another important function of Antibodies is promoting the phagocytosis. This mechanism is called opsonization. The Phagocytes recognize the Fc region of an Antibody molecule which is bound on the surface of a pathogen.

The third capacity of antibodies is the activation of the complement system. Antibodies coating the surface of a pathogen can be recognized by the complement proteins which have different functions. Complement proteins can promote the phagocytosis or lysis of the pathogen.

Antibodies were produced by B cells. B cells are lymphocytes and belong to the adaptive immunity and emanate from the common lymphoid progenitors under the impact of IL-7.  They undergo two different gene rearrangement (D-J and V-DJ rearrangement)  before they express the B cell receptor (BCR) in the immature phase.

The BCR is specific to one epitope. If a epitope of a pathogen binds to the BCR, the B cell internalize and degrade them and bind the degraded particles to MHC II molecules. A CD4+ T-cell (Th2 cell response) binds to another epitope of the pathogen with the t cell receptor (TCR). The CD4 receptor binds coevally to the MHC II molecule. Both the MHC II:CD4 and the BCR:TCR signal and also the binding of CD40 to the CD40L are important for the activation of the T cell. This activation leads to the secretion of different cytokines from the T cell. IL-4, IL-5 and IL-6 leads to the proliferation and differentiation of the B cells.

The CD40:CD40L complex and different cytokine patterns leads also to an antigen class switch from IgM to IgG, IgE or IgA. After teh activation can the b cell differentiate to a plasma cell or to a memory b cell, which can produce faster and more IgG after a recurring infection.

[1] Murphy, K. et al. Janeway’s Immunobiology, 7th edition. Garland Science 2008

DC’s – different subtypes for different tasks

Dendritic cells are an impotant part of the immune system and also important for connecting the adaptive with the innate immune system. They are so-called antigen presenting cells, have the ability to take up pathogens and present fragments of them to CD4+ t cells on the MHC-II-molecule.

Dendritic cells stem from different progenitors. One class arise from myeloid the other from lymphoid progenitors. There are different subtypes of each class which have different skills.

There are two different main subgroups of dendritic cells descending from different progenitors. CMP (common myeloid progenitors) and CLP (common lymphoid progenitors) and are located in different parts of the body. The CMP are located in the bone marrow whereas CLP are in the lymphoid organs.

Different Subtypes arise from these progenitors with different abilities. CMP can differentiate to interstitial DC or to Langerhans DC, wheras plasmacytoid DC arise from CLP. It is common that monocytes can differentiate to monocyte-derived DC when monocytes are stimulated with GM-CSF and IL-4.

The different subtypes are characterized by different surface molecules and receptors. They define the different tasks of each subtypes and the localisation in the body.

In the next Blog I will discuss the different tasks of the different subtypes considering the different surface molecules and their interaction within the tissue.

 - Murphy K. et al. Janeway’s Immunobiology 7th Edition, Garland Science Publisher

- Ken Shortman and Yong-Jun Liu.  Mouse and Human Dendritic Cell Subtypes. Nature Review Immunology 151-161 (2002)

Dendritic cells – The link between innate and adaptive immunity

“DC are important cells of the immune system. They are phagocytes and have the ability to take up pathogens. On their surface are different receptors of the pattern recognition receptors family (PRR). On the one hand the TLR (Toll like receptors) Family are included. On the other hand are receptors like C-type lectins (e.g. DC-SIGN) and mannose binding receptors present on the cell surface. The signals vary strongly after binding of a pathogen on the different PRR’s. Interestingly, the different receptors of the TLR family recognize different pattern from different parts of a pathogen. If a Toll like receptor binds a pathogen the intracellular cascade starts. This leads to an activation of NF-KB and the production of different pro-inflammatory cytokines (e.g: IL1, IL12, TNFa,…).

The Binding of  known structures of a pathogen to a C-type lectin results in a phagocytosis. Phagocytosis is a process in which pathogens will take up by endocytosis. During this process the endosome will fusion with a primary lysosme to a secondary lysosome. Within the lysosome hypochloric acid and enzymes digest pathogens to fragments. In the lysosomal membrane integrated MHC-II molecules will specifically bind to the fragments and migrate intracellulary to the cell membrane (the foreign fragment will be shown on the outside of the cell). The presenting fragment can activate t cells, which are important for the humoral immune response.

The uptake or binding of a pathogen leads to the activation of dendritic cells. They loose their ability to take up pathogens und begin to produce cytokines and chemokines and migrate to the secondary lymph nodes for the activation of t-cells.

Dendritic cells are able to induce the innate immune response. They produce pro-inflammatory cytokines and chemokines, which leads to the migration of phagocytes (neutrophils, monocytes,…). They play also a key role in the humoral immune response. DC are capable antigen-presenting cells (APC) and show their MHC-II:fragment complex to CD4+ t-cells. These t-cells are important for the activation of b-cells and the production of antibodies.”

I tried to write a post on english for practice. The post could include some mistakes and I apologise for them.

Immunobiology, Charles Janeway, 6th edition, Garland Science