A Novel Mechanism for Complement Activation at the Surface of B Cells Following Antigen Binding

Crystal structure of duck egg lysozyme isoform II (DEL-II)

Background

Lysozyme purified from duck eggs (DEL) has long been used as a model antigen as a counterpoint to the enzyme purified from hen eggs (HEL). However, unlike the single C-type variant found in hen eggs, duck eggs contain multiple isoforms: I, II and III. We recently reported the structures of isoforms I and III from Pekin duck (Anas platyrhynchos) and unequivocally determined the sequences of all three isoforms by mass spectrometry. Here we present the crystal structure of isoform II (DEL-II).

Results

Lysozyme isoform II was purified from isoforms I and III using ion-exchange and gel-filtration chromatography, then crystallized. X-ray diffraction data were collected to 1.15 Å resolution and the structure of DEL-II was solved by molecular replacement using the structure of DEL-I as the search model. It contains two molecules in the crystallographic asymmetric unit: both molecules display a canonical C-type lysozyme fold and electron density consistent with the expected sequence. The most significant difference between the two molecules concerns different conformations of a surface loop containing one of the expected amino acid differences between the isoforms.

Conclusions

The structure of DEL-II supports the primary sequence as elucidated by a combination of amino acid sequencing, DNA sequencing and mass spectrometry, with strong electron density confirming it to be an S37G G71R variant of DEL I, and differing from hen egg lysozyme at a total of 21 amino acid positions.

Electronic supplementary material

The online version of this article (10.1186/s12900-018-0090-7) contains supplementary material, which is available to authorized users.

Structural basis of antigen recognition: crystal structure of duck egg lysozyme

Duck egg lysozyme (DEL) is a widely used model antigen owing to its capacity to bind with differential affinity to anti-chicken egg lysozyme antibodies. However, no structures of DEL have so far been reported, and the situation had been complicated by the presence of multiple isoforms and conflicting reports of primary sequence. Here, the structures of two DEL isoforms from the eggs of the commonly used Pekin duck (Anas platyrhynchos) are reported. Using structural analyses in combination with mass spectrometry, non-ambiguous DEL primary sequences are reported. Furthermore, the structures and sequences determined here enable rationalization of the binding affinity of DEL for well documented landmark anti-lysozyme antibodies.

Defective B cell ontogeny and immune response in human complement receptor 2 (CR2, CD21) transgenic mice is partially recovered in the absence of C3

Mice prematurely expressing human CR2 (hCR2) in the B cell lineage have a defective B cell ontogeny and immune response. Our recent analysis of this phenotype suggested that signaling through hCR2 and presumably mouse CD19 on the B cell surface, during bone marrow development, could result in the observed changes in B cell function in these mice. To test this hypothesis, we back crossed hCR2(high) transgenic mice onto the CD19(-/-) background. CD19(-/-)hCR2(high) mice were found to possess even fewer mature B cells than their CD19(+/+)hCR2(high) littermates, demonstrating that loss of CD19 exacerbated the effects elicited through hCR2. This data suggests that CD19 provides a survival signal during B cell development in this model. Next, we examined if the removal of the main ligand for CR2, namely C3d, through back-crossing onto the C3(-/-) background could restore normal B cell development. However, we found only minor recovery in peripheral B cell numbers and no obvious change in function. This was despite a three-fold increase in the level of hCR2 expression on B cells isolated from the spleen or bone marrow of C3(-/-)hCR2(high) mice when compared with C3 sufficient littermates. These data demonstrate that hCR2 is integrated in mouse B cell signaling and that the downstream effects of hCR2 expression during early B cell development are partially but not completely due to interaction with C3 fragments and signaling through CD19 in the bone marrow environment.