{{short description|Fragment}} 300px|thumb|Rotating scFv fragment with highlighted complementarity determining regions (CDRs) [[File:Single chain variable fragment.svg|thumb|The two possible structures of a single-chain variable fragment, with the antigen binding sites including the N-termini on the left and the C-termini on the right. The linker peptides are shown as arrows.]] A '''single-chain variable fragment''' ('''scFv''') is not actually a fragment of an antibody, but instead is a fusion protein of the variable regions of the heavy (V<sub>H</sub>) and light chains (V<sub>L</sub>) of immunoglobulins, connected with a short peptide linker of ten to about 25 amino acids.<ref>{{cite journal | last1 = Huston | first1 = J. S. | last2 = Levinson | first2 = D. | last3 = Mudgett-Hunter | first3 = M. | last4 = Tai | first4 = M. S. | last5 = Novotný | first5 = J. | last6 = Margolies | first6 = M. N. | last7 = Crea | first7 = R. | year = 1988 | title = Protein engineering of antibody binding sites: recovery of specific activity in an anti-digoxin single-chain Fv analogue produced in Escherichia coli | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 85 | issue = 16| pages = 5879–5883 | doi=10.1073/pnas.85.16.5879| pmid = 3045807 | pmc = 281868 | bibcode = 1988PNAS...85.5879H | doi-access = free }}</ref>
Each variable fragment contains <u>framework regions (FWRs)</u>, providing stability to the structure, and <u>complementarity-determining regions (CDRs)</u>, which are the regions responsible for antigen recognition<ref name=":0">{{Cite journal |last=Chailyan |first=Anna |last2=Marcatili |first2=Paolo |last3=Tramontano |first3=Anna |date=2011-06-28 |title=The association of heavy and light chain variable domains in antibodies: implications for antigen specificity |url=https://doi.org/10.1111/j.1742-4658.2011.08207.x |journal=The FEBS Journal |volume=278 |issue=16 |pages=2858–2866 |doi=10.1111/j.1742-4658.2011.08207.x |issn=1742-464X}}</ref>.
The linker is usually rich in glycine, which provides flexibility, as well as serine or threonine, that provides solubility. The peptide linker can either connect the N-terminus of the V<sub>H</sub> with the C-terminus of the V<sub>L</sub>, or ''vice versa''<ref name="Schirrmann">{{cite thesis|url=http://edoc.hu-berlin.de/dissertationen/schirrmann-thomas-2005-03-18/HTML/|language=de|last=Schirrmann|first=Thomas|title=Tumorspezifisches Targeting der humanen Natürlichen Killerzellinie YT durch Gentransfer chimärer Immunglobulin-T-Zellrezeptoren|date=8 November 2004|location=Berlin|doi=10.18452/15246}}</ref>, but changes in the orientation of the V<sub>H</sub>, peptide linker and V<sub>L</sub> have been described to affect scFv’s affinity and specificity<ref name=":0" />.
This protein retains the specificity of the original immunoglobulin, despite removal of the constant regions and the introduction of the linker.<ref name="Peterson" /> The image to the right shows how this modification usually leaves the specificity unaltered.
These molecules were created to facilitate phage display, where it is highly convenient to express the antigen-binding domain as a single peptide. As an alternative, scFv can be created directly from subcloned heavy and light chains derived from a hybridoma. ScFvs have many uses, e.g., flow cytometry, immunohistochemistry, and as antigen-binding domains of CAR-T cells.
Unlike monoclonal antibodies, which are often produced in mammalian cell cultures, scFvs are more often produced in bacteria cell cultures such as ''E. coli''.<ref name="Peterson">{{cite journal|title=Monoclonal Antibody Form and Function: Manufacturing the Right Antibodies for Treating Drug Abuse|last1=Peterson|first1=Eric|journal=AAPS Journal|year=2006|volume=8|issue=2|pages=E383–E390|url=http://www.aapsj.org/view.asp?art=aapsj080243|doi=10.1208/aapsj080243|pmid=16796389|last2=Owens|first2=SM|last3=Henry|first3=RL|pmc=3231570}}</ref>
==Purification== Single-chain variable fragments lack the constant Fc region found in complete antibody molecules, and, thus, the common binding sites (e.g., protein G) cannot be used to purify antibodies. These fragments can often be purified or immobilized using protein L, since protein L interacts with the variable region of kappa light chains. More commonly, scientists incorporate a six histidine tag on the c-terminus of the scFv molecule and purify them using immobilized metal affinity chromatography (IMAC). Some scFv can also be captured by protein A if they contain a human VH3 domain.<ref name="Vostakolaei 14711–14724">{{Cite journal |last1=Vostakolaei |first1=Mehdi Asghari |last2=Molavi |first2=Ommoleila |last3=Hejazi |first3=Mohammad Saeid |last4=Kordi |first4=Shirafkan |last5=Rahmati |first5=Saman |last6=Barzegari |first6=Abolfazl |last7=Abdolalizadeh |first7=Jalal |date=September 2019 |title=Isolation and characterization of a novel scFv antibody fragments specific for Hsp70 as a tumor biomarker |url=https://onlinelibrary.wiley.com/doi/10.1002/jcb.28732 |journal=Journal of Cellular Biochemistry |language=en |volume=120 |issue=9 |pages=14711–14724 |doi=10.1002/jcb.28732 |pmid=30998271 |s2cid=121351794 |issn=0730-2312|url-access=subscription }}</ref><ref>{{Cite journal|url=https://pubmed.ncbi.nlm.nih.gov/9186782-staphylococcal-protein-a-binding-to-vh3-encoded-immunoglobulins/|pmid = 9186782|year = 1997|last1 = Potter|first1 = K. N.|last2 = Li|first2 = Y.|last3 = Pascual|first3 = V.|last4 = Capra|first4 = J. D.|title = Staphylococcal protein a binding to VH3 encoded immunoglobulins|journal = International Reviews of Immunology|volume = 14|issue = 4|pages = 291–308|doi = 10.3109/08830189709116521}}</ref><ref>{{Cite journal |last1=Kordi |first1=Shirafkan |last2=Rahmati-Yamchi |first2=Mohammad |last3=Asghari Vostakolaei |first3=Mehdi |last4=Barzegari |first4=Abolfazl |last5=Abdolalizadeh |first5=Jalal |date=2019-02-21 |title=Purification of a Novel Anti-VEGFR2 Single Chain Antibody Fragmentand Evaluation of Binding Affinity by Surface Plasmon Resonance |url=https://apb.tbzmed.ac.ir/Abstract/apb-22975 |journal=Advanced Pharmaceutical Bulletin |language=en |volume=9 |issue=1 |pages=64–69 |doi=10.15171/apb.2019.008 |issn=2228-5881 |pmc=6468230 |pmid=31011559}}</ref>
==Bivalent and trivalent scFvs== thumb|300px|Structure of divalent (top) and trivalent (bottom) scFvs, tandem (left) and di-/trimerisation format (right) ''Divalent'' (or ''bivalent'') single-chain variable fragments (di-scFvs, bi-scFvs) can be engineered by linking two scFvs. This can be done by producing a single peptide chain with two V<sub>H</sub> and two V<sub>L</sub> regions, yielding ''tandem scFvs''.<ref>{{cite journal|title=Development of tumor targeting anti-MUC-1 multimer: effects of di-scFv unpaired cysteine location on PEGylation and tumor binding|first5=SJ|last5=Denardo|first4=GL|last4=Denardo|first3=XB|last3=Shi|first2=A|last2=Natarajan |last1=Xiong|first1=Cheng-Yi|journal=Protein Engineering Design and Selection|year=2006|volume=19|issue=8|pmid=16760193|pages=359–367|doi=10.1093/protein/gzl020|doi-access=free}}</ref><ref>{{cite journal|title=A revival of bispecific antibodies|first1=Peter|last1=Kufer|first2=Ralf|last2=Lutterbüse|first3=Patrick A.|last3=Baeuerle|journal=Trends in Biotechnology|volume=22|issue=5|pages=238–244|year=2004|url=http://www.micromet.de/fileadmin/template/main/pdf/publications_147881aaf61df52304237f0ee7f0cf2a.pdf|doi=10.1016/j.tibtech.2004.03.006|pmid=15109810}}</ref> Another possibility is the creation of scFvs with linker peptides that are too short for the two variable regions to fold together (about five amino acids), forcing scFvs to dimerize. This type is known as ''diabodies''.<ref>{{cite journal|pmid=8341653|title="Diabodies": small bivalent and bispecific antibody fragments|last1=Hollinger|first1=Philipp|volume=90|issue=14|pmc=46948|date=July 1993|pages=6444–8|last2=Prospero|first2=T|last3=Winter|first3=G|journal=Proceedings of the National Academy of Sciences of the United States of America|doi=10.1073/pnas.90.14.6444|bibcode=1993PNAS...90.6444H|doi-access=free}}</ref> Diabodies have been shown to have dissociation constants up to 40-fold lower than corresponding scFvs, meaning that they have a much higher affinity to their target. Consequently, diabody drugs could be dosed much lower than other therapeutic antibodies and are capable of highly specific targeting of tumors ''in vivo''.<ref name="Adams">{{cite journal|pmid=9652755|year=1998|last1=Adams|first1=GP|last2=Schier|first2=R|last3=McCall|first3=AM|last4=Crawford|first4=RS|last5=Wolf|first5=EJ|last6=Weiner|first6=LM|last7=Marks|first7=JD|title=Prolonged in vivo tumour retention of a human diabody targeting the extracellular domain of human HER2/neu|volume=77|issue=9|pages=1405–12|pmc=2150193|journal=British Journal of Cancer|doi=10.1038/bjc.1998.233}}</ref> Still shorter linkers (one or two amino acids) lead to the formation of trimers, so-called ''triabodies'' or ''tribodies''. ''Tetrabodies'' have also been produced. They exhibit an even higher affinity to their targets than diabodies.<ref>{{cite journal|title=Di-, tri- and tetrameric single chain Fv antibody fragments against human CD19: effect of valency on cell binding|first4=M|last4=Little|first3=G|last3=Moldenhauer|journal=FEBS Letters|first2=SM|volume=453|last2=Kipriyanov|issue=1|pages=164–168|first1=F.|last1=Le Gall|pmid=10403395|doi=10.1016/S0014-5793(99)00713-9|year=1999|bibcode=1999FEBSL.453..164L|s2cid=20213440|doi-access=free}}</ref>
All of these formats can be composed from variable fragments with specificity for two different antigens, in which case they are types of bispecific antibodies.<ref>{{cite journal|pmid=11388794|year=2001|last1=Dincq|first1=S|last2=Bosman|first2=F|last3=Buyse|first3=MA|last4=Degrieck|first4=R|last5=Celis|first5=L|last6=De Boer|first6=M|last7=Van Doorsselaere|first7=V|last8=Sablon|first8=E|title=Expression and purification of monospecific and bispecific recombinant antibody fragments derived from antibodies that block the CD80/CD86-CD28 costimulatory pathway|volume=22|issue=1|pages=11–24|doi=10.1006/prep.2001.1417|journal=Protein Expression and Purification}}</ref><ref>{{cite thesis|first1=C|last1=Kellner|url=http://www.opus.ub.uni-erlangen.de/opus/volltexte/2009/1235/|title=Entwicklung und Charakterisierung bispezifischer Antikörper-Derivate zur Immuntherapie CD19-positiver Leukämien und Lymphome|trans-title=Development and characterisation of bispecific antibody derivatives for the immunotherapy of CD19-positive leukaemia and lymphoma|language=de, en|publisher=Friedrich-Alexander-Universität|location=Erlangen-Nürnberg|year=2008}}</ref> The furthest developed of these are bispecific tandem di-scFvs, known as bi-specific T-cell engagers (BiTE antibody constructs).
==Examples== * Pexelizumab, a scFv binding to component 5 of the complement system and designed to reduce side effects of cardiac surgery<ref>{{cite journal|pmid=15331798|year=2004|last1=Mathew|first1=JP|last2=Shernan|first2=SK|last3=White|first3=WD|last4=Fitch|first4=JC|last5=Chen|first5=JC|last6=Bell|first6=L|last7=Newman|first7=MF|title=Preliminary report of the effects of complement suppression with pexelizumab on neurocognitive decline after coronary artery bypass graft surgery|volume=35|issue=10|pages=2335–9|doi=10.1161/01.STR.0000141938.00524.83|journal=Stroke: A Journal of Cerebral Circulation|doi-access=free}}</ref> * C6.5, a diabody targeting HER2/neu<ref name="Adams" /> found in some breast cancers * Brolucizumab, a scFV binding to VEGF-A and used to treat wet age-related macular degeneration * '''G6A''', a scFV binding to human Hsp70.1 and can be used to target this protein as a potential marker in vast cancers.<ref name="Vostakolaei 14711–14724"/>
==References== {{reflist|2}}
{{Engineered antibodies}}
{{DEFAULTSORT:Single-Chain Variable Fragment}} .