When I’m not postdoc-ing, as part of my job I’m also involved with teaching at the Doctoral Training Centre here in Oxford. I mainly teach the first-year students of the Systems Approaches to Biomedical Science CDT – many members of this group are doing (or have done) their DPhils through this program (including myself!). Recently, I and some other OPIGlets were responsible for two modules called Structural Biology and Structure-Based Drug Discovery, and as part of those modules we arranged a practical session on 3D printing.

Most of the time, the way we ‘see’ protein structures is through a computer screen, using visualisation software such as PyMOL. While useful, these virtual representations have their limitations – since the screen is flat, it’s difficult to get a proper feel for the structure¹, and seeing how your protein could interact and form assemblies with others is difficult. Physical, three-dimensional models, on the other hand, allow you to get hands-on with your structure, and understand aspects of your protein that couldn’t be gained from simply looking at images. Plus, they look pretty cool!

This year, I printed three proteins for myself (shown in the photo above). Since my most recent work has focused on transmembrane proteins, I felt it was only right to print one – these are proteins that cross membranes, usually to facilitate the transport of molecules in and out of the cell. I chose the structure of a porin (top of the photo), which (as the name suggests) forms a pore in the cell membrane to allow diffusion across it. This particular protein (1A0S) is a sucrose-specific porin from a type of bacteria called Salmonella typhimurium, and it has three chains (coloured blue, pink and purple in the printed model), each of which has a beta barrel structure. You can just about see in the photo that each chain has regions which are lighter in colour – these are the parts that sit in the cell membrane layer; the darker regions are therefore the parts that stick out from the membrane.

My second printed model was the infamous Zika virus (bottom right). Despite all the trouble it has caused in recent years, in my opinion the structure of the Zika virus is actually quite beautiful, with the envelope proteins forming star-like shapes in a highly symmetrical pattern. This sphere of proteins contains the viral RNA. The particular structure I used to create the model (5IRE) was solved using cryo-electron microscopy, and required aligning over 10,000 images of the virus.

Finally, I printed the structure of a six-residue peptide, that’s probably only interesting to me… Can you tell why?!²

1 – However, look at this link for an example of looking at 3D structures using augmented reality!

2 – Hint: Cysteine, Leucine, Alanine, Isoleucine, Arginine, Glutamic Acid…

Last week my boyfriend Ben Rainthorpe returned from Argentina having successfully climbed Aconcagua – the highest mountain in South America. At a staggering 6963m above sea level it is the highest peak outside of the Himalayas. The climb took 20 days in total with a massive 14 hours of hiking and climbing on summit day.

Aconcagua is part of the mountaineering challenge known as the Seven Summits. This is achieved by summiting the highest mountain in each of the seven continents. This was first successfully completed in 1985 by Richard Bass. In 1992 Junko Tabei became the first woman to complete the challenge. In December Ben quit his job as a primary teacher to follow his dream of achieving this feat. Which mountains constitute the seven summits is debated and there are a number of different lists. In addition the challenge can be extended by including the highest volcano in each continent.

The Peaks:

1.Kilimanjaro – Africa (5895m) 

Kilimanjaro is usually the starting point for the challenge. At 5895 m above sea level and no technical climbing required it is a good introduction to high altitude trekking. However, this often means it is underestimated and the most common cause of death on the mountain is altitude sickness.

2. Aconcagua – South America (6963 m)

The next step up from Kilimanjaro Aconcagua is the second highest of the seven summits. However the lack of technical climbing required make it a good second peak to ascend after Kilimanjaro. For Aconcagua however, crampons and ice axes are required. The trek takes three weeks instead of one.

3. Elbrus – Europe (5,642 m)

Heralded as the Kilimanjaro of Europe, Elbrus even has a chair lift part of the way up! This mountain is regularly underestimated causing a high number of fatalities per year. Due to snowy conditions crampons and ice axes are once again required. Some believe that Elbrus should not count as the European peak and instead Mount Blanc should be summited – a much more technical and dangerous climb.

4. Denali – North America (6190 m).

Denali is a difficult mountain to summit. Although slightly lower than other peaks, the distance from the equator means the effects of altitude are more keenly felt. More technical skills are needed. In addition there are no porters to help carry additional gear so climbers must carry a full pack and drag a sled.

5. Vinson Massif – Antartica (4892 m).

Vinson is difficult because of the location rather than any technical climbing. The costs of going to Antartica are great and the conditions are something to be battled with.

6. Puncak Jaya – Australasia (4884 m) or Kosciuszko – Australia (2228 m)

The original Seven Summits included Mount Kosciuszko of Australia – the shortest and easiest climb on the list. However it is now generally agreed that Puncak Jaya is the offering from the Australasia continent. Despite being smaller than others on the list this is the hardest of the seven to climb with the highest technical rating. It is also located in an area that is highly inaccessible to the public due to a large mine, and is one of the few where a rescue by helicopter is not possible.

7. Everest – Asia (8848 m).

Everest is the highest mountain in the world at 8848 m above sea level. Many regard the trek to Everest Base Camp as challenge enough. Some technical climbing is required as well as bottled oxygen to safely reach altitudes of that level. One of the most dangerous parts is the Khumbu Icefall which must be traversed every time the climbers leave base camp. As of 2017 at least 300 people have died on Everest – most of their bodies still remain on the mountain.

Ben has now climbed two of the Seven Summits. His immediate plans are to tackle Elbrus in July (which I might try and tag along to) and Vinson next January. If you are interested in his progress check out his instagram (@benrainthorpe).

Back-to-back posting – I wanted to talk about the growing volume of TCR structures in the PDB. A couple of weeks ago, I presented my database to the group (STCRDab), which is now available at http://opig.stats.ox.ac.uk/webapps/stcrdab.

Unlike other databases, STCRDab is fully automated and updates on Fridays at 9AM (GMT), downloading new TCR structures and annotating them with the IMGT numbering (also applies for MHCs!). Although the size of the data is significantly smaller than, say, the number of antibody structures (currently at 3000+ structures and growing), the recent approval of CAR therapies (Kymriah, Yescarta), and the rise of interest in TCR engineering (e.g. Glanville et al., Nature, 2017; Dash et al., Nature, 2017) point toward the value of structures.

Feel free to read more in the paper, and here are some screenshots. 🙂

STCRDab front page.

Look! 5men, literally.

Possibly my new favourite PDB code.

STCRDab annotates structures automatically every Friday!

Currently I’m working on developing a new strategy to use FREAD within the ABodyBuilder pipeline. While running some tests I’ve realised that some of the RMSD values that there were some minor miscalculations of CDR loops’ RMSD in my paper.

To start with, the main message of the paper remains the same; the overall quality of the models (Fv RMSD) was correct, and still is. ABodyBuilder isn’t necessarily the most accurate modelling methodology per se, but it’s unique in its ability to estimate RMSD. ABodyBuilder would still be capable of doing this calculation regardless of what the CDR loops’ RMSD may be. This is because the accuracy estimation looks at the RMSD data and places a probability that a new model structure would have some RMSD value “x” (given the CDR loop’s length). Our website has now been updated in light of these changes too.

Update to Figure 2 of the paper.

Update to Figure S4 of the paper.

Update to Figure S5 of the paper.

There are nearly 17,000 Oxford students on taught courses. They turn up reliably every October. We send them to an army of lecturers and tutors, drawn from every rank of the research hierarchy. As members of that hierarchy, we owe it to the students – all 17,000 of them – to teach them as best we can.

And where can we learn the most about how to teach? There are 438,000 professional teachers in the UK. Maybe people who spend all of their working time on the subject might have good strategies to help people learn.

The context of the paper

Teachers obsess over assessment. Assessment is the process by which teachers figure out what students have learned. It is probably true that assessment is the only reason we have classrooms at all.

Inside the Black Box is of the vanguard of recent changes in educational thinking. Modern teaching regards good pedagogy as a practical skill. Like other types of performance, it depends on a specific set of concrete actions which can be taught and learned. Not everyone is a natural teacher – but nearly everyone can become a competent teacher.

Formative assessment is the focus of Inside the Black Box. The article argues that this process, in which teachers figure what students know and tell them how it’s going wrong, is essential to good classroom practice.

What is the black box?

The black box is the classroom. After societal convulsions over class sizes, funding deficits, curriculum reforms, and examination structure, it’s time – says the article, in 2001 – that we focus on what actually goes on inside the classroom. These social changes, it says, adjust the inputs to the black box, and society expects better things out of the black box. But what if changing the inputs makes the work inside the black box harder? Don’t we have an obligation to figure out what needs to happen to get students to learn?

The article touches three questions:

• Is there evidence that improving formative assessment raises standards?
• Is there evidence that there is room for improvement?
• Is there evidence about how to improve formative assessment?

The answers are yes, yes, and yes. In meta-analyses of educational experiments, formative assessment consistently raises standards. These experiments match the experience of teachers, who know that the least effective lessons are those which do not respond to students’ needs. Standard observations – such as those from Ofsted – ask teachers to answer what are they learning, and then how do you know, and then what are you doing about it?

The second question – is there room for improvement? – is one they address in great detail in the context of primary and secondary education. Some criticisms (the giving of grades for its own sake, unintentional encouragement of “rote or superficial learning”, relentless competition between students) seem applicable in different parts of our university context. A greater weakness is a lack of emphasis. People engaged in university teaching frequently center the delivery of knowledge instead of learning, an idea exacerbated by our obsession with lectures and masked by the long lag between those lectures and the exams in which we assess them.

Recommendations

Inside the Black Box makes specific recommendations for instructors about how to engage in formative assessment. Those recommendations – unusually, for an item in the educational literature – are specific and detailed. But rather than focus on them, it is worth examining three themes which run across the article.

The overriding focus is the importance of formative assessment. If we care about what students learn, then we’ve got to be checking what it is that they actually are learning. Opportunities for formative assessment should be “designed into any piece of teaching”. In extremis, this idea has interesting implications for the institution of lectures, which generally lack them entirely.

A subsidiary idea is the importance of setting clear objectives for learning. Too many students view learning as a series of exercises rather than a step in the formation of a coherent body of knowledge. The overarching direction should be made clear. And on a more detailed level, we need to be explicit about what outcomes we want our students to obtain so that they know whether they are making satisfactory progress. Formative assessment must make reference to expectations, and formative self- or peer assessment becomes impossible if those expectations are not well-understood.

And this discussion ties into a final point: when students truly apply themselves to the task of learning, their self-perception and self-esteem becomes bound up in it. Ineffective expectation-setting and insufficient clarity about the means for improvement result in students feeling demotivated, which causes them to revise their goals downward. They put in less effort and achieve outcomes that are worse. These effects are costly and can be avoided by effective formative assessment.

Inside the Black Box is a diversion from our diet of scientific articles, but I think it is worth our attention. Pedagogy is difficult to get right. In the university context, good practice is the subject of little attention and rarely assessed. Thinking about good asssessment means that our students benefit.

But all communication activities are a form of teaching. Really good teachers communicate really well. When good communication happens, everyone benefits, inside and outside the black box.

Typography [tʌɪˈpɒɡrəfi]
    n.: the style and appearance of printed matter.

Perhaps a “glossed” feature of making graphs, having the right font goes a long way. Not only do we have the advantage of using a “pretty” font that we like, it also provides an aesthetic satisfaction of having everything (e.g. in a PhD thesis) in the same font, i.e. both the text and graph use the same font.

Fonts can be divided into two types: serif and sans-serif. Basically, serif fonts are those where the letters have little “bits” at the end; think of Times New Roman or Garamond as the classic examples. Sans-serif fonts are those that lack these bits, and give it a more “blocky”, clean finish – think of Arial or Helvetica as a classic example.

Typically, serif fonts are better for books/printed materials, whereas sans-serif fonts are better for web/digital content. As it follows, then what about graphs? Especially those that may go out in the public domain (whether it’s through publishing, or in a web site)?

This largely bottles down to user preference, and choosing the right font is not trivial. Supposing that you have (say, from Google Fonts), then there are a few things we need to do (e.g. make sure that your TeX distribution and Illustrator have the font). However, this post is concerned with how we can use custom fonts in a graph generated by Matplotlib, and why this is useful. My favourite picks for fonts include Roboto and Palatino.

The default font in matplotlib isn’t the prettiest ( I think) for publication/keeping purposes, but I digress…

To start, let’s generate a histogram of 1000 random numbers from a normal distribution.

The default font in matplotlib, bitstream sans, isn’t the prettiest thing on earth. Does the job but it isn’t my go-to choice if I can change it. Plus, with lots of journals asking for Type 1/TrueType fonts for images, there’s even more reason to change this anyway (matplotlib, by default, generates graphs using Type 3 fonts!). If we now change to Roboto or Palatino, we get the following:

Sans-serif Roboto.

Serif font Palatino.

Basically, the bits we need to include at the beginning of our code are here:

# Need to import matplotlib options setting method
# Set PDF font types - not necessary but useful for publications
from matplotlib import rcParams
rcParams['pdf.fonttype'] = 42

# For sans-serif
from matplotlib import rc
rc("font", **{"sans-serif": ["Roboto"]}

# For serif - matplotlib uses sans-serif family fonts by default
# To render serif fonts, you also need to tell matplotlib to use LaTeX in the backend.
rc("font", **{"family": "serif", "serif": ["Palatino"]})
rc("text", usetex = True)

This not only guarantees that images are generated using a font of our choice, but it gives a Type 1/TrueType font too. Ace!

Happy plotting.

What is the “biological space” and why is this space so important for all researchers interested in developing novel drugs? In the following, I will first establish a definition of the biological space and then highlight its use in computationally developing novel drug compounds and as a starting point in the experimental exploration of biological systems.

While chemical space has been defined as the entirety of all possible chemical compounds which could ever exist, the definition of biological space is less clear. In the following, I define biological space as the area(s) of chemical space that possess biologically active (”bioactive”) compounds for a specific target or target class¹. As such, they can modulate a given biological system and subsequently influence disease development and progression. In literature, this space has also been called “biologically relevant chemical space”².

Only a small percentage of the vast chemical space has been estimated to be biologically active and is thus relevant for drug development, as randomly searching bioactive compounds in chemical space with no prior information resembles the search for “the needle in a haystack”. Hence, it should come as no surprise that bioactive molecules are often used as a starting point in in-silico explorations of biological space.
The plethora of in-silico methods for this task includes similarity and pharmacophore searching methods^3-6 for novel compounds, scaffold-hopping approaches to derive novel chemotypes^7-8 or the development of quantitative structure-activity relationships (QSAR)^9-10 to explore the interplay between the 3D chemical structure and its biological activity towards a specific target.

The biological space is comprised of small molecules which are active on specific targets. If researchers want to explore the role the role of targets in a given biological system experimentally, they can use small molecules which are potent and selective towards a specific target (thus confided to a particular area in chemical space)^11-12.
Due to their high selectivity ( f.e. a greater than 30-fold selectivity towards proteins of the same family¹²), these so-called “tool compounds” can help establish the biological tractability – the relationship between the target and a given phenotype – and its clinical tractability – the availability of biomarkers – of a target¹¹. They are thus highly complementary to methods such as RNAi, CRISPR¹² and knock-out animals¹¹. Consequently, tool compounds are used in drug target validation and the information they provide on the biological system can increase the probability of a successful drug ¹¹. Most importantly, tool compounds are particularly important to annotate targets in currently unexplored biological systems and thus important for novel drug development¹³.

1. Sophie Petit-Zeman, http://www.nature.com/horizon/chemicalspace/background/figs/explore_b1.html, accessed on 03.07.2016.
2. Koch, M. A. et al. Charting biologically relevant chemical space: a structural classification of natural products (SCONP). Proceedings of the National Academy of Sciences of the United States of America 102, 17272–17277 (2005).
3. Stumpfe, D. & Bajorath, J. Similarity searching. Wiley Interdisciplinary Reviews: Computational Molecular Science 1, 260–282 (2011).
4. Bender, A. et al. How Similar Are Similarity Searching Methods? A Principal Component Analysis of Molecular Descriptor Space. Journal of Chemical Information and Modeling 49, 108–119 (2009).
5. Ai, G. et al. A combination of 2D similarity search, pharmacophore, and molecular docking techniques for the identification of vascular endothelial growth factor receptor-2 inhibitors: Anti-Cancer Drugs 26, 399–409 (2015).
6. Willett, P., Barnard, J. M. & Downs, G. M. Chemical Similarity Searching. Journal of Chemical Information and Computer Sciences 38, 983–996 (1998)
7. Sun, H., Tawa, G. & Wallqvist, A. Classification of scaffold-hopping approaches. Drug Discovery Today 17, 310–324 (2012).
8. Hu, Y., Stumpfe, D. & Bajorath, J. Recent Advances in Scaffold Hopping: Miniperspective. Journal of Medicinal Chemistry 60, 1238–1246 (2017)
9. Cruz-Monteagudo, M. et al. Activity cliffs in drug discovery: Dr Jekyll or Mr Hyde? Drug Discovery Today 19, 1069–1080 (2014).
10. Bradley, A. R., Wall, I. D., Green, D. V. S., Deane, C. M. & Marsden, B. D. OOMMPPAA: A Tool To Aid Directed Synthesis by the Combined Analysis of Activity and Structural Data. Journal of Chemical Information and Modeling 54, 2636–2646 (2014).
11. Garbaccio, R. & Parmee, E. The Impact of Chemical Probes in Drug Discovery: A Pharmaceutical Industry Perspective. Cell Chemical Biology 23, 10–17 (2016).
12. Arrowsmith, C. H. et al. The promise and peril of chemical probes. Nature Chemical Biology 11, 536–541 (2015).
13. Fedorov, O., Müller, S. & Knapp, S. The (un) targeted cancer kinome. Nature chemical biology 6, 166–169 (2010).

I couldn’t decide whether to write this blog post. However, I sifted through the archives of BLOPIG and found in the original post this excerpt:

“And if your an athlete, like Anna (Dr. Lewis) who crossed the atlantic in a rowing boat or Eleanor who used to row for the blues – what can I say, this is how we roll, or row [feeble attempt at humour] – thats a non-scientific but unique and interesting experience too (Idea #8).  .”

Therefore I’ve decided that it might be an interesting post to look into what life is like when you are studying for a DPhil and also training for the blues. Rowing in particular is a controversial sport – I have heard of many stories advocating that rowing will be the absolute detriment to your DPhil. I’ve never felt pressured as part of OPIG to give up rowing – all of my supervisors have been very fair, in that if I get the work done then they accept this is part of my life. However, I realise all supervisors are not so understanding. I hope this blog post will give some insight into what it is like to trial for a Blues sport (in this case Women’s Lightweight Rowing), whilst studying for a DPhil at Oxford.

4:56 am – Alarm goes off. If its after September it’s dark, cold and likely raining. No breakfast as I will do the first training session fasted – just get dressed and go!

5:15 am – Leave the house with a bag full of kit, food for the day, laptop and papers to cycle to Iffley Sport’s Centre

5:45 am – Lightweight Women’s minibus leaves from Iffley to drive to Wallingford. Some girls try to study in the bus, but to be honest its too dark and we’re all a bit too sleepy.

6:15 am – Arrive at Wallingford. Get onto the water for a session in the boats. Although in the Boat Race we race in an 8 (8 rowers with one oar each, with a cox steering), we spend lots of time in different boats throughout the season. Perhaps unlike our openweight counterparts, we also do a lot of sculling (two oars per rower) as the only Olympic class boat for lightweight women is a sculling boat. We travel to Wallingford for a much longer, emptier stretch of river and normally get to see the sunrise.

8:10 am – We leave Wallingford to head back to Oxford. Start waiting in A LOT of traffic once you hit the ring road, and there’s a lot of panic in the bus about whether 9 am lectures will be made on time!

8:50 am – Arrive back at Iffley Sport’s Centre. Grab bike and cycle to the department.

9:00-9:15 am – Arrive at the Department. Quick shower to thaw frozen fingers and to not repulse my fellow OPIG members. I then get to eat warm porridge (highlight of the day) and go through my emails. I also check whether any of my jobs have finished on the group servers – one of the great perks of being in OPIG is the computational resources available to the group. Check the to-do list from yesterday and write a to-do list for today and get to work (coding, plotting results, reading papers or writing)!

11:00 am (Tuesdays & Thursdays) – Coffee morning! Although if it’s any time close to a race no bourbon biscuits or cake for me. This is a bit of an issue because at OPIG we eat a lot of cake. However, one member can usually be relied upon to eat my portion..

1:00 pm – Lunchtime! As a lightweight rower I am required to weigh-in at 59kg on the day of the Boat Race. If I am over that weight I don’t get to race. Therefore, I spend a portion of the year dieting to make sure I hit that target. The dieting lunch consists of soup and Greek yogurt. The post race non-dieting lunch consists of pasta from Taylors, chocolate and a Coke (yum!). OPIG members generally all have lunch at this time and enjoy solving the Times Cryptic Crossword. I’m not the best at crosswords so I normally chat to Laura and don’t concentrate.

2:00 pm – Back to work. Usually coding whilst listening to music. I normally start rushing to be able to submit some jobs to the group servers before I have to leave the office.

3:00 pm – Go to get a chocomilk with Clare. A chocomilk from the vending machine in our department costs 20p and is only 64 calories!

5:30 pm – Cycle to Iffley Sports Centre for the second training session of the day.

5:45 pm – If it’s light enough we hop in the minibus to go to Wallingford for another outing on the water. However, for most of the season its too dark and we head to the gym. This will either consist of weights to build strength, or we will use the indoor rowing machine (erg) to build fitness. The erg is my nemesis, so this is not a session I look forward to. Staring at a screen that constantly tells you how hard you are pushing, or if you are no longer pushing as hard I find to be psychologically quite tough. I’d much rather be gliding along the river.

8:35 pm – Leave Iffley after a long session to head home. Quickly down a Yazoo (strawberry milk) to boost recovery as I won’t be eating dinner until 45 minutes to an hour after the end of the session.

9:00 pm – Arrive home. I “cook” dinner which when I’m dieting consists of chucking sweet potato and healthy sausages from M&S in the oven while I pack my kit bag for the next day.

9:30 pm – Wolf down dinner and drink about a pint of milk, whilst finally catching up with my boyfriend about both our days.

10:00 pm – Bedtime at the latest.

Repeat!

In my last presentation, I talked about the article by King et al. describing a method for computationally removing T-cell receptor epitopes from proteins. The work could have significant impact on the field of designing protein therapeutics, where immunogenicity is a serious obstacle.

One of the major challenges when developing a protein therapeutic is the activation of the immune system by the drug and subsequent production of antibodies against it, rendering the therapeutic ineffective. This process is known as immunogenicity. Immunogenicity is triggered by T-cells recognition of peptide epitopes displayed on the MHC (major histocompatibility complex). This recognition can be impeded by designing the protein therapeutic to remove the potential T-cell epitopes from its surface. There has been some success in experimental T-cell epitope removal, but the process remains resource and time consuming.

In this work, King et al. created a function which assigns to each residue a score that measures its propensity to be a part of a T-cell epitope. The score consists of three parts. The first part is based on a SVM (Support Vector Machine) score calculated over each 15-residue long window, that attempts to predict how likely is the corresponding peptide sequence to bind the MHC. The SVM has been trained on the available immunological data from the Immune Epitope Database (IEDB). The second part of the score is calculated on each 9-residue window and compares the frequency of the 9-mer in the host genomic data and in the known epitope data (a sequence occurring with a high frequency in a human genome would be rewarded while the opposite is true for sequences occurring in the known epitope data). The third part penalizes any deviations from the original charge of the protein. These three parts are combined with a standard Rosetta score that measures the stability of the protein. The weights assigned to each segment were calibrated on existing protein structures. The combined score would be used to score the mutations in the sequence of the protein of interest, according to their propensity of reducing immunogenicity. The top scoring mutations would then be combined in a greedy fashion.

The authors tested their method on fluorescent reporter protein superfolder GFP (sfGFP) and the toxin domain of the cancer therapeutic HA22. In the case of sfGFP the authors targeted the four top-scoring T-cell epitopes. They created eight different proteins designs, out of which all preserved the function of the original protein (fluorescence). The authors selected the top scoring design for experimental immunogenicity testing. The experiments have shown that the selected design had a significantly reduced immunogenicity in comparison to the original protein. In the case study of HA22 the authors created five designs, out of which three displayed cytotoxicities at the same level or higher than the original protein. The two most cytotoxic designs were further characterized experimentally for their propensity to induce immune response. The authors have found that the two mutants elicited a significantly reduced T-cell response.

Figure 1: Reduction of immunogenicity without loss of function. A) Three of the five designs show cytotoxicity at the same level or higher than the original protein. B) Two of the three cytotoxicity-preserving designs show reduced immunogenicity

Overall, this very interesting study showed that computational methods can be successfully used for reducing immunogenicity of protein therapeutics, opening new avenues for computational protein design.

This blogpost is be about the “Computational design of an epitope-specific Keap1 binding antibody using hotspot residues grafting and CDR loop swapping” by Liu et. al. that I presented at group meeting in May.

Antibody design is a subject that I am closely interested in, especially methods that have an important computational step. So far the go-to methods for designing an antibody used by industry are animal model immunisation and/or phage display, with little or no use of computational methods. In the past few years, however, a few computational methods for rational design of antibodies have been making a showing. Firstly, there are the ones where a structure of the docked antibody-antigen already exists, and the antibody is further refined computationally to increase binding affinity. Then there are the ones where the paratope of the antibody is proposed by the designer against a specific target. The paper I am summarizing here by Liu et. al follows the latter idea in a neat way.

Liu et. al. show that if a specific motif is important for binding a certain target, i.e. there is a crystal structure which shows that the motif is buried in the target and/or you predict that its residues are important for binding, it is worthwhile trying to graft that that motif in the CDR area of antibody (the one which is responsible for antibody specificity and affinity). Grafting of entire CDR loops has been long used for antibody humanisation, with many examples where CDR loops maintaining conformation and binding specificity when being transferred from a non-human scaffold to a human scaffold. This is somewhat  aided by the fact that the starting and end points of the area being grafted is stable (i.e. the anchors are  conformationally the same in all the antibody structures that we observe), which is not the case in Liu et al where they graft a four residue motif. The cool thing they do which makes it more probable for the motif to maintain conformation is identify an antibody which has in one of its CDR loops a fragment with the same backbone conformation with the motif they are trying to graft.  They then just replace the residue types to the ones that are known to bind the target. For the Nrf2 motifs (that binds Keap1) they managed to create 5 potential designs. These were further expanded, using rational point mutations on the rest of the antibody in order to increase possibility of binding, to 10. Out of the 10 two showed binding.

One of the potential issues in a real scenario however is the fact that not an entire binding site is copied on antibody, the motif being a subset of the whole, which means the possibility of a low affinity and/or low chances of competing with the original protein (i.e. Nrf2) from which the motif was copied. This actually turned out to be the case, with the initial designs showing low mM affinity. Liu et. al. further worked on improving the initial designs, and they did so by computationally swapping the H3 CDR of the initial designs to a set of other H3 structures that have been seen in other solved antibodies using the Rosetta design protocol. They retained the ones that had a predicted buried SASA of > 2000 A^2, a change in energy of more than 20 REU and a shape complementarity greater than 0.6. These were then tested experimentally with a few of them showing nM affinities, a result which at this time should make you very happy if your entire design phase was done computationally.