Immunoglobulins (2 of 2) Ali Al Khader, MD Faculty of Medicine Al-Balqa’ Applied University Email: ali.alkhader@bau.edu.jo

Save this image in your brain Visit https://en.wikipedia.org/wiki/Immunoglobulin_heavy_chain for references

Generation of antibody diversity Germline heavy and light chain gene segments VDJ or VJ -Recombination -Transcription -Translation Heavy and light chain polypeptides Immunoglobulin assembly Modified from immunology for medical students 2nd edition Gene segments will be rearranged during B cell development to assemble a functional gene of either a light or heavy chain

Gene segments L leader V D diversity J joining C The variable region 4 subtypes of lambda due to 4 different constant region versions For heavy chain Constant region Each gene segment is composed of a set or group of versions of that segment, e.g., Jk (joining kappa) gene set has 5 different versions of Jk See Kuby immunology for references

Synthesis of light chain After removal of introns (splicing) from primary RNA transcript Leader peptide is removed by proteolytic enzymes after translation modified

Synthesis of heavy chain All these steps are before transcription L L Then: transcription and splicing of the RNA primary transcript…and the leader peptide will be removed after translation, along with glycosylation L What will happen in splicing? L L L Visit https://pt.wikipedia.org/wiki/Recombina%C3%A7%C3%A3o_V(D)J for references..modified

The recombination is somatic (not germline), but when does it occur ???

Factors of antibody diversity Germline diversity…due to the different versions of each segment …51 VH,27 DH, 6 JH , 40 V kappa, 5 J Kappa, 30 V lambda and 4 J lambda Combinatorial diversity If we want to synthesize an Ig with kappa light chain for example

Factors of antibody diversity, cont’d Junctional diversity: …In the recombination process, we delete some nucleotides and add others (e.g., the random addition by TdT enzyme) at junction sites between the adjoined segments Somatic hypermutation…see next slide

Somatic hypermutation = affinity maturation Due to: very high-rate point mutations in the V regions of heavy and light chains …will produce antibodies with higher affinity for the antigen …B cells expressing these antibodies are preferentially selected for becoming plasma cells

Allelic exclusion There is genetic polymorphism in the genes of VH, CH, VL, and CL …which means that there’s difference in each heterozygous individual between the allele from the mother and that from the father …in heterozygotes for these allotypes, each B cell either express genes from the mother or from the father (not from both)…= allelic exclusion *but the population of B cells in the same person is mixed

Heavy chain isotype switching (Ig class switching) When memory B cells are restimulated, they undergo further DNA rearrangement to juxtapose their already-rearranged VDJ gene regions to different heavy chain C region genes Variable regions are not changed here …so: the specificity for the antigen is the same

Heavy chain isotype switching (Ig class switching) Variable region is the same Visit http://www.pinsdaddy.com/immunoglobulin-gene-rearrangement_fmB1dE*pw71srNWRpFMIiGXeqLxSN4Et9LeFQHwkL98ZmahBBbQF4HLmEJIuNvLm9l%7CnIebWb2sdAlSyRfYNiQ/2 for references...modified

This sequence is important Lippincott’s illustrated reviews immunology 2nd edition..modified

How does the unstimulated B cell express both IgM and IgD?

Important enzymes V(D)J recombinases -For cleaving and rejoining DNA segments -RAG-1 and RAG-2 only found in lymphocytes (if absent: problem in lymphocyte development) = recombination activating gene Terminal deoxynucleotidyl transferase (TdT) …adds nucleotides

Thank You