The influence of hapten spacer arm length on antibody response and immunoassay development

Antibodies against small molecules with high titer and high affinity are always pursued in the field of vaccines for drugs of abuse, antidotes to toxins and immunoassays in medical, environmental, and food safety. The exposure degree of the target molecule to the immune system is critical to induce a strongly specific antibody response, thus, the spacer arm length between the target molecule and carrier protein plays an important role. However, the influence of spacer arm length on antibody titer, affinity, and assay performance is not yet clear and highly demanded to be addressed. In the present study, we proposed a model study to answer the question by using two typical small molecules, melamine and p-nitroaniline, which were introduced by varied spacer arms with increasing alkane linear length from 2 to 12 carbon atoms brick by brick. The spacer arm lengths of the haptens were obtained by computational chemistry. The titer and affinity of mouse antisera were analyzed and compared, showing that all haptens with spacer arms of 6-8 carbon atoms, i.e. 6.3-8.8 Å in length, induced strong antibodies represented by the highest titer and affinity without exception, while the haptens with spacer arms of 2-4 carbon atoms and 10-12 carbon atoms, i.e. 1.5-3.9 Å and 11.3-13.9 Å in length, failed to induce high-quality antibody response. Moreover, the titer and sensitivity of the subsequently developed immunoassays were significantly affected by using coating haptens with different spacer arm lengths, and coating haptens with a spacer arm of 6.3-8.8 Å in length delivered the optimum detection performance. The antibody recognition mechanism study further confirmed that the hapten spacer arm length had a critical effect on the recognition properties of the induced antibody, which should be interactive with the spacer arm each other. This study showed that the hapten with appropriate spacer arm length is important to antibody response and immunoassay development, providing a valuable and general clue for the rational design of hapten.