The profile of Ruslan medzhitov, a pioneer in Immunology

I came across the profile of Ruslan Medzhitov in HHMI website. I am reproducing this for the readers. His passion for science and immunology makes him break many myths in Immunology.
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Sometimes a little sweet talk goes a long way. A silver tongue catapulted Ruslan Medzhitov from Tashkent to Yale, where he's helped revolutionize our understanding of the immune system.
The early 1990s were a bleak time for science in the Soviet Union. As the empire crumbled, scientific resources drained away, until just a single battered copy of the weekly journals made the rounds at Moscow University. As a graduate student there, Medzhitov yearned to keep up with the latest advances, and his weekly hour with Science and Nature wasn't enough. So he headed to the Academy of Natural Sciences, which was then engaged in its own detente with the university. For various bureaucratic reasons, university students weren't allowed access to the library. "So I had to go and flirt with the librarians—there were several of them—and eventually they all knew me and let me in secretly and told me not to tell anyone," says Medzhitov.
There, in the stacks, the young biology student stumbled on a copy of Cold Spring Harbor Symposia. In it was the paper that launched his career. Written by the late Yale immunologist Charles Janeway (an HHMI investigator), the article sketched a new theory for how the immune system recognizes and responds to pathogens. Little was known then about the so-called innate immune system and how it identifies and reacts to invaders. Janeway's ideas ignited Medzhitov, sending him to his university's sole e-mail terminal. "I was able to send messages once a week," says Medzhitov. "And my first message was to Charlie." Medzhitov asked the professor for more details about his ideas. To Medzhitov's delight, Janeway responded, and the pair exchanged several more messages.
"Charlie's paper was the only paper that made sense of a lot of things," says Medzhitov. "That was the point I first thought about being a researcher in immunology. As an undergraduate student, I never had a course on immunology."
With a career path now in mind, Medzhitov landed a fellowship at the University of California, San Diego. There, working with protein evolution pioneer Russell Doolittle, Medzhitov contacted local immunologist Richard Dutton, who knew Janeway and recommended Medzhitov for a postdoctoral position in Janeway's lab. Janeway said yes. "I felt very lucky," says Medzhitov.
When he arrived at Yale—after a detour to Moscow to defend his thesis and sweat out a government coup and six months of uncertainty—Medzhitov felt overwhelmed. "Janeway's lab was very famous, and I imagine competition to get in was very high. And I was coming from just a few e-mail exchanges and a recommendation. My challenge was, not only did I not speak English well, I also had never done any experiments. In Russia, there was no money to do anything. All I could do was sit in the library. So I arrived without any experience, basically zero. I had to learn as quickly as I could."
It turns out that lack of experience helped Medzhitov in another way. Janeway's theory of how innate immunity acted, by recognizing bits of invading organisms, was "extremely speculative." And that meant it was risky to work on. But, being "oblivious to concerns about career," Medzhitov jumped in on the project. "I was just happy to be in a place where I could do science," he says.
In 1996, after just a few years working together, Janeway and Medzhitov made a breakthrough. They discovered receptors that alerted the second arm of the immune system, the more familiar T cells and B cells that attack pathogens. Studying these proteins, dubbed Toll-like receptors, quickly became one of the hottest areas in biology. "That was an extremely exciting time," says Medzhitov. "We didn't realize how much would come out of it eventually, that it would become such a huge area of research."
In the decade-plus since then, Medzhitov has piled one discovery after another upon the first, dramatically expanding our understanding of the key roles Toll-like receptors play in infection control, chronic inflammation, and even the growth of tumors. At the same time, he's branched off in a dozen directions: One example of many, Medzhitov is learning how commensal bacteria—which live in our guts and help us digest food—also help protect our intestines from injury.
Medzhitov now thinks that Toll-like receptors and related proteins may trigger the chronic inflammation that leads to coronary artery disease, Alzheimer's, and diabetes—some of our biggest killers. "I like a lot of areas of biology and it's hard for me to focus on only one," he says. Now, with plenty of journals to read and experiments to conduct, he doesn't have to.

Dr. Medzhitov is also Professor of Immunobiology at Yale School of Medicine.

Th9 mystery: Is it finally solved?

In the recent article by Kaplan group, the authors have dissected the mechanisms by which Th9 cells develop. The Th9 cells are described as CD4+IL9+IL10+ T cells by Stockinger and Kuchroo groups in nature Immunology 2008. Previously, immunologists felt that IL9 secretion is part of Th2 function. IL9 is an important cytokine secreted by Th2 cells which has an effect on mast cell function. However, there is ambiguity in secretion of IL9 by Th2 polarised cells.
Kaplan group have identified PU.1 as the transcription factor that drives Th9 polarisation. PU.1 is expressed by sub-population of Th2 populations expressing low IL4. In addition, PU.1 deficient cells secrete less IL9 compared to the cells that have PU.1. The authors also show that IL10 secretion is not characteristic feature of Th9 cells as reported by Stockinger and Kuchroo groups before. The authors identify the chromatin modifications on il9 locus  by PU.1 by ChIP assays. Finally, by analyzing allergic patients and also the allergic asthma model, the authors show the obligatory role of  IL9 and PU.1 in the manifestation of asthma.
By this study, the authors have made the knowledge of Th cells more complex. The T cells are now known to be plastic. The regulatory T cells and the Th17 cells share common but contrasting developmental pathways. Similiarly, Th2 cells develop in presence of IL4 while the Th9 cells need IL4 and TGFb1 for their development.Also, PU.1 and GATA-3 interact with each other similar to FoxP3 and RORg which answers the plasticity among the Th subsets.These raise important issues about the role of Th cells in innate immune responses where there is need for spontaneous immune responses.

Recommended reading:
1. H. C. Chang, S. Sehra, R. Goswami, W. Yao, Q. Yu, G. L. Stritesky, R. Jabeen, C. McKinley, A. N. Ahyi, L. Han, E. T. Nguyen, M. J. Robertson, N. B. Perumal, R. S. Tepper, S. L. Nutt, and M. H. Kaplan. The transcription factor PU.1 is required for the development of IL-9-producing T cells and allergic inflammation. Nat.Immunol., 2010.

2.V. Dardalhon, A. Awasthi, H. Kwon, G. Galileos, W. Gao, R. A. Sobel, M. Mitsdoerffer, T. B. Strom, W. Elyaman, I. C. Ho, S. Khoury, M. Oukka, and V. K. Kuchroo. IL-4 inhibits TGF-beta-induced Foxp3+ T cells and, together with TGF-beta, generates IL-9+ IL-10+ Foxp3(-) effector T cells. Nat.Immunol. 9 (12):1347-1355, 2008. 

3.W. Elyaman, E. M. Bradshaw, C. Uyttenhove, V. Dardalhon, A. Awasthi, J. Imitola, E. Bettelli, M. Oukka, Snick J. Van, J. C. Renauld, V. K. Kuchroo, and S. J. Khoury. IL-9 induces differentiation of TH17 cells and enhances function of FoxP3+ natural regulatory T cells. Proc.Natl.Acad.Sci.U.S.A 106 (31):12885-12890, 2009. 

4. E. E. Forbes, K. Groschwitz, J. P. Abonia, E. B. Brandt, E. Cohen, C. Blanchard, R. Ahrens, L. Seidu, A. McKenzie, R. Strait, F. D. Finkelman, P. S. Foster, K. I. Matthaei, M. E. Rothenberg, and S. P. Hogan. IL-9- and mast cell-mediated intestinal permeability predisposes to oral antigen hypersensitivity. J.Exp.Med. 205 (4):897-913, 2008.

Altered regulatory T cells: A consequence of mast cells in colorectal cancer

I justed came across this paper by Blatner et al. Colorectal cancer is characterised by abundant Tregs and mast cells. However, mechanisms on how the tumor is able to survive in spite of such atmosphere is unclear. Here the authors study human CRC and murine polyposis tumors. The authors report systemic skewing of Treg from anti- to proinflammatory functions in both human CRC and murine polyposis; which they call "altered Tregs". MC are target of suppression by Treg but at the same time play a critical role to reverse the anti-inflammatory function of the Treg. They predict that targeting the pathogenic cross-talk between Treg and MC to allow recovery of Treg antiinflammatory functions will help to control CRC.
First, the  authors show that cytokine milieu in the tumors show increased IL17, TGFβ, IL6, and IL1β with corresponding reductions in IL10 and INFγ; these favor Th17 polarization.They then show that there is increased FoxP3 Tregs and mast cells in the colorectal cancer tumors. These tregs from patient tumors are unable to suppress mast cell degranulation, lack IL10 but show increased IL17 when compared to healthy donors. Then, the authors show that IL6 enables the diversion of healthy human Tregs to a pro-inflammatory phenotype with ability to inhibit CD4 T cells. The authors even generate pSTAT3+ rpo-inflammatory Tregs by co-culturing of naive Treg with primary MC. However, by performing bone marrow transplantation experiments, the authors show that IL6 is sufficient but not absolutely necessary for Treg conversion.Thus, MC induce Treg to switch function and escalate inflammation in CRC without losing T-cell–suppressive properties without the need of  IL6 and IL17 in this process.


Suggested Reading:
1. Blatner et al, In colorectal cancer mast cells contribute to systemic regulatory T-cell dysfunction, PNAS 2010,  107 (14), 6430-6435 
2. Gri G, et al. CD4+CD25+ regulatory T cells suppress mast cell degranulation and allergic responses through OX40-OX40L interaction. Immunity 2008, 29:771–781. 
3. Piconese S, et al. Mast cells counteract regulatory T cell suppression through interleukin-6 and OX40/OX40L axis toward Th17 cell differentiation. Blood 2009, 114:2639–2648.

 

Catalytic antibodies in hemophilia patients

This is the PhD project of my friend Bharath Wootla. It's interesting how his mentor Sébastien Lacroix-Desmazes guided by Srinivas Kaveri got this idea, and published in Nature Medicine, and a much bigger study later on in the New England Journal of Medicine. The idea is simple: congenital hemophilia patients lack the gene for factor VIII, a protein that is pivotal in blood coagulation pathway. When patients suffering from hemophilia undergo an injury, their blood does not clot and can lead to death due to bleeding. One way of treating these people is to administer exogenous FVIII. However, the immune system of some patients consider the exogenous FVIII as foreign and develop anti-FVIII antibodies. These antibodies inhibit the procoagulant activity of FVIII and hence are also known as FVIII inhibitors. Sébastien in 1999 showed that these antibodies possess catalytic activity and Bharath later on from 2004-2009 studied the mechanisms by which these antibodies inactivate FVIII. The enzymatic activities were studied by incubating the patient's IgG with FVIII. The resultant mixture was analyzed using SDS-PAGE gels for the hydrolysis of FVIII. They have used intravenous immunoglobulin, a pool of IgG from 1000's of healthy blood donors as a control along with the molecules incubated alone in the buffer to check for auto-degradation of proteins. Due to my limited knowledge of enzyme kinetics, I would suggest the readers to refer the publications listed below for more information.

Suggested Reading:
1. Lacroix-Desmazes S, Moreau A, Sooryanarayana, Bonnemain C, Stieltjes N, Pashov A, Sultan Y, Hoebeke J, Kazatchkine MD, Kaveri SV. Catalytic activity of antibodies against factor VIII in patients with hemophilia A. Nat Med. 1999 Sep;5(9):1044-7.
2. Lacroix-Desmazes S, Wootla B, Dasgupta S, Delignat S, Bayry J, Reinbolt J, Hoebeke J, Saenko E, Kazatchkine MD, Friboulet A, Christophe O, Nagaraja V, Kaveri SV. Catalytic IgG from patients with hemophilia A inactivate therapeutic factor VIII. J Immunol. 2006 Jul 15;177(2):1355-63.
3. Wootla B, Mahendra A, Dimitrov JD, Friboulet A, Borel-Derlon A, Rao DN, Uda T, Borg JY, Bayry J, Kaveri SV, Lacroix-Desmazes S. Factor VIII-hydrolyzing IgG in acquired and congenital hemophilia. FEBS Lett. 2009 Aug 6;583(15):2565-72.

The orign of IKDC (Interferon producing killer dendritic cells)

The discovery of IKDC or Interferon producing Dendritic Cells in 2006 created enthusiasm in the immunology research (refrences 1 and 2). The DCs that can kill! what a link between innate and adaptive immunity. Interferons are involved in elimination of  viruses etc that are engulfed in the phagocytes. These interferons are secreted by NK or natural killer cells. The IKDCs were put along with other "hybrid cells" such as NKT cells, which share features with natural killer (NK) cells and T lymphocytes, and plasmacytoid dendritic cells (DCs), which combine the qualities of B cells and conventional DCs. IKDCs can function as NK cells by killing the cells lacking MHC molecules, like plasmacytoid DCs that produce interferon and like DCs that present antigen to conventional T cells. IKDCs not only secret type I and type II interferons to recognize and kill tumor cells effectively, but also express MHC-II molecules to present antigens. Thus, IKDCs are considered as important immunosurveilance cells for tumors, providing a link between innate and adaptive immunity. However, the developmental pathways of these cells were not fully understood.
However, Di Santos group from Pasteur Institute, Paris and Caminschi group from Australia studied the development of IKDCs and proved that these are not DCs but actually a subset of NK cells (Refrence 3 and 4). Generally mature NK cells in humans or mice express NK group 2, member D (NKG2D), CD161, NK-cell protein 46 (NKp46) and CD122. However, the expression of Ly49 family members, CD127, CD27 and KLRG1 (killer-cell lectin-like receptor subfamily G, member 1) in mice, and of KIRs (killer-cell immunoglobulin-like receptors), CD56 and CD16 in humans suggests heterogeneity within the mature NK-cell pool.


Suggested reading:
1. Chan et al, Interferon-producing killer dendritic cells provide a link between innate and adaptive immunity, Nature Medicine - 12, 207 - 213 (2006)
2. Taieb et al, A novel dendritic cell subset involved in tumor immunosurveillance, Nature Medicine - 12, 214 - 219 (2006)
3. Vosshenrich et al, CD11cloB220+ interferon-producing killer dendritic cells are activated natural killer cells.J Exp Med. Oct 29;204(11):2569-78 (2007)
4. Caminschi et al, Putative IKDCs are functionally and developmentally similar to natural killer cells, but not to dendritic cells.J Exp Med. Oct 29;204(11):2579-90 (2007)
5. Huntington et al, Developmental pathways that generate natural-killer-cell diversity in mice and humans, Nature Reviews Immunology 7, 703-714 (2007)

Mystery mast cells under scanner again

Mast Cells are the most neglected immune cells to be studied yet are the most important amongst them. They are the classical mediators of allergic reactions. When the antigen-specific IgE binds to an antigen (usually allergen), it triggers mast cell activation by binding to high-affinity Fc epsilon receptor I. This leads to the release of mast cell mediators such as histamine, chymotryptase, tryptase, TNF etc. To protect the mast cells from activation induced death, the CD200R is induced. CD200R inhibits mast cell activation through ITIM (immunoreceptor tyrosine-based inhibition motif ).

Recent evidence shows that Mast cellos sense pathogens through toll-like receptor mediated pathways. In addition, the receptors for IgG2a and Ig light chains have been identified. However, the receptor for IgLC is yet to be characterised. Added to this is the concept of MCregs (regulatory mast cells). Mast cells are identified in the triumvirate along with Tregs and Th17. While the tregs inhibit mast cells, mast cells in turn activate the Th17 cells. The complete mechanisms underlying this three-way interactions is to be explore.

Basophils: The neglected minority gains a new respect as antigen presenting cells

To function as an antigen-presenting cell, the cell must fulfill a number of criteria. Upon antigen exposure, the candidate cell must express MHC class II and co-stimulatory molecules. It must be able to take up and process antigen and localize to draining lymph nodes, where efficient T cell–antigen-presenting cell interactions take place. Ideally, the candidate cell would also express the relevant cytokines required for T cell differentiation, although other accessory cells may provide the required cytokines. Finally, the candidate cell must be able to induce CD4⁺ T cell proliferation and differentiation in in vitro and in vivo assays.
Differentiation of naïve CD4⁺ T cells into Th2 cells requires three signals: T cell receptor triggering through peptide antigen recognition in the context of MHC class II molecule, amplification of T cell receptor signaling via co-stimulatory molecules and the presence of appropriate cytokine. For Th2 cell differentiation, however, dendritic cells are not the one-stop shop for all three required signals since dendritic cells do not produce IL-4, the cytokine necessary for Th2 cell differentiation. This observation implies that other antigen-presenting cells, in addition to dendritic cells, may contribute to Th2 cell differentiation.
The studies by Artis and colleagues (Perrigoue et al, Nat Immunol 2009), Medzhitov and colleagues (Sokol al, Nat Immunol 2009), and Nakanishi and colleagues (Yoshimoto al, Nat Immunol 2009), use three different models and elegantly demonstrate that basophils meet the criteria for antigen-presenting cells for Th2 cell differentiation.
Artis and colleagues studied the intestinal helminth parasite Trichuris muris. They showed that CD11c-restricted expression of MHC class II was not sufficient to generate a Th2 inflammatory response. Medzhitov and colleagues investigated the role of basophils in Th2 response to protease antigen papain and OVA. They demonstrated that MHC expression on basophils was sufficient to drive Th2 cell differentiation. Nakanishi and colleagues focused on the role of basophils in augmentation of Th2 responses by antigen–IgE immune complexes.
With basophils added to the list of cells capable of antigen presentation to naïve CD4⁺ T cells, the question arises as to what is the dominant antigen-presenting cell in Th2 inflammatory responses. Four approaches were used to highlight the dominant role of basophils.
First, the authors showed that basophil depletion through treatment with a monoclonal antibody to FceRI significantly diminished Th2 cell differentiation. The second approach used diphtheria toxin in CD11c-DTR mice and demonstrated that depletion of dendritic cells did not alter Th2 cell differentiation in response to OVA plus papain Th2 immunity to T. muris. The third approach demonstrated that CD11c-restricted expression of MHC class II was not sufficient to induce Th2 cell differentiation in response to OVA plus papain or to protect against infection with T. muris. In the fourth approach, mice were immunized in the ear with papain and the ear was either left intact or removed after two hours in order to remove the source of tissue resident dendritic cells. The authors observed Th2 cell differentiation, albeit at lower levels, in the draining lymph node, despite removal of skin resident dendritic cells.
The idea that the basophil is the dominant antigen-presenting cell for Th2 cell differentiation is certainly intriguing, but its validation requires further investigation.

What discriminates danger signals from pathogens with that of damage to tissues

The immune system protects against infection by pathogenic microorganisms, but it also recognizes when the body has been injured. Burns, radiation exposure, and bruises all involve the immune system when responding to damaged tissue. Which signaling pathways recognize tissue damage and keep the resulting inflammatory response from getting out of hand? Patten recognition receptors, which recognize pathogens or components of injured cells (danger), trigger activation of the innate immune system. Whether and how the host distinguishes between danger- versus pathogen-associated molecular patterns remains unresolved. In seminal paper in Science (Vol. 323. no. 5922, pp. 1722 - 1725, March 2009), Chen and colleagues demonstrated an unique mechanisms by which the body differentiates between self- and non-self signals during danger.
They that CD24-deficient mice exhibit increased susceptibility to danger- (DAMPs) but not pathogen-associated molecular patterns (PAMPs). CD24 associates with high mobility group box 1 (HMGB1), heat shock protein 70 (HSP70), and heat shock protein 90 (HSP90), negatively regulates their stimulatory activity, and inhibits nuclear factor-kappa B (NF-B) activation. This occurs at least in part through CD24 association with Siglec-10 in humans or Siglec-G in mice. Our results reveal that the CD24-Siglec-G pathway protects the host against a lethal response to pathological cell death and discriminates danger- versus pathogen-associated molecular patterns. Through association with and inhibition of the molecules that are released after tissue damage, CD24 and Siglec-G protected mice from an otherwise lethal inflammatory response.