We thank staff members of the beamlines for his or her help with X-ray diffraction data collection. conformation. Our results suggest a masking mechanism to explain how HIV-1 shields this critical region from the human being immune system. Keywords: C4, CD4, Env, HIV-1, monoclonal antibody Intro The HIV-1 envelope glycoprotein (Env) gp120 initiates viral access into sponsor cells by binding to its receptor CD4 and to its co-receptor CCR5/CXCR4, and it is the major target for acquired immune deficiency syndrome vaccine development. However, gp120 uses many decoys to evade immune surveillance in humans, rendering the development of a protecting vaccine very demanding. Conformational masking, by either JZL184 covering immunogenic epitope areas with additional domains, or by having them adopt different conformations, is one of the decoys gp120 uses to evade the immune reactions.1,2 For example, variable loops can often adopt different conformations, and antibodies that recognize one conformation will not be able to effectively target another conformation.3,4,5 Conformational masking can also guard functionally conserved sites within gp120. The CD4 receptor-binding site is definitely safeguarded by entropy masking,1 and the co-receptor-binding site in the pre-fusion complex is completely buried under variable loops.6,7,8 CD4 receptor binding will expose the co-receptor binding site, which is comprised of various conserved regions including the fourth conserved region (C4). The C4 region of gp120, which consists of residues 416-4599 (HxB2 numbering10), offers many important practical roles. For example, it is directly involved in receptor binding, co-receptor binding and co-receptor selection (tropism).11,12 Crystal constructions of gp120 complexes have revealed that residues 425 (Asn), 426 (Met), and 427 (Trp) in the C4 region have direct contact with CD4.13 The C4 region, together with the third variable loop (V3), is also involved in co-receptor binding. Early mutagenesis studies indicated that residues 438 (Pro) and 441 (Gly) in the C4 region are important for CCR5 binding.14 Structural studies of gp120 in complex with CD4 and monoclonal antibody (mAb) 412d showed that residues 439 (Ile), 440 (Arg), and 441 (Gly) in the C4 region are involved in binding with the N-terminus of CCR5.6 A slight conformational modify in the C4 region can influence the structure of V3, and even a single amino acid mutation in the C4 region can increase the neutralization sensitivities of anti-V3 antibodies.15,16 The C4 region is JZL184 also involved in co-receptor selection, and mutations of residue 440 in the C4 region can alter co-receptor JZL184 specificity.17 The C4 region is highly immunogenic. It can induce cell-mediated immunities in HIV-1 infected individuals and in immunized animals.18,19 For example, monomeric gp120 can elicit mouse helper T-cell immune responses reactive having a C4 peptide, named T1 (a 16-mer containing the region of residues 428-443).18 The C4 region can also induce humoral immune responses.20,21 In fact, the CD4 binding region of gp120 was first identified by an anti-C4 mAb, 5C2E5, which was raised by immunizing mice having a recombinant gp120, JZL184 and its epitope region was identified by competition with CD4 binding.11 Since then, several antibodies targeting the C4 region have been generated in animals, including rabbit polyclonal antibodies R10-12 and R19-21 that were raised having a poliovirus chimaera expressing a region of JZL184 17 amino acids of C4,22 mouse mAbs G3-42, G3-299, G3-508, and G3-536 that were raised having a recombinant BH10 gp120,23,24 and rat mAbs ICR 38.8f and ICR38.1a that were raised with the recombinant BH10 gp120.25 One of the characteristics of these antibodies is that they can block CD4 binding of gp120, and thus, they were collectively named CD4-blocking antibodies. 26 The C4 region was initially suggested to form amphipathic helices;19 however, crystal structures of CD4-bound gp120 molecules have shown that it actually forms two beta strands, numbered 20 and 21, of the bridging sheet and loop F.13 Strand 20 is involved in CD4 binding, while strand 21 is involved in co-receptor binding. The constructions of gp120 in Rabbit polyclonal to Caspase 4 complex with numerous mAbs showed the C4 region could display.
