An innate natural antibody is reactive with a cryptic sequence of lactoferrin exposed on sperm head surface

Lactoferrin (LF), an 80-kDa glycoprotein of ubiquitous occurrence in body fluids, is multifunctional and capable of assuming different configurations to serve those functions. The capacity of LF to undergo endocytosis and the recent demonstration of LF binding to sequence specific DNA indicate that a function or capability of LF, in addition to iron chelation, bacteriostasis, and receptor-specific lymphocyte binding, may be that of gene activation or silencing. The data of this report present a human physiological system, that of sperm entry into the oocyte in performance of fertilization in which, since LF is a component of the sperm protein coat, that capability could be expressed. However, the configuration of LF in that locus is one in which a revealed cryptic sequence provides the specific binding site for a natural antibody present in the fertilization milieu. The presence of that antibody suggests that a system of control of the potential interaction of LF with the intra-ooplasmic DNA, that of gametes or pronuclei, is operative. The configuration of LF on the sperm surface and designation of the reactive site for the natural antibody were enabled by a monoclonal antibody secreted by a hybridoma derived from a human cord blood B cell. Thus, in addition to information concerning the molecular flexibility of LF, these observations support the proposition that the repertoire of natural antibodies provides an innate homeostatic system, with each antibody serving a specific role.