The Landscape of Neutralizing antibodies (NAb): Production, Mechanisms of Action (MOA), FDA approved-antibodies, and Functional assay

Antibodies are made by B-cells in the bone marrow. Bone marrow has been long thought to be a hematopoietic organ. When B-cells are created, they begin to produce antibodies that will bind to specific antigens. Antigen-specific antibody producing, long-term lived plasma cells are largely found in the bone marrow, which contributes to humoral immune responses.

Affifinity maturation is based on the somatic mutation of the germline genes of immunoglobulin, a process called somatic hypermutation (SHM), which consists of point mutations that are performed by a protein called activation-induced cytosine deaminase (AID), resulting in a greater affifinity of antibodies or, in the worst cases, a decrease in affifinity.

This process is carried out in germinal centers (GCs), specialized microstructures formed in secondary lymphoid tissues upon infection or immunization, producing longlived plasma cells and memory B cells, which protect against reinfection. GCs are organized into two regions, the dark zone (DZ) and the light zone (LZ).

In the DZ, there are B cells with a high proliferation rate (called centroblasts) and SHM. The centroblasts then enter the LZ (now are called centrocytes), where they capture and process antigens present on follicular dendritic cells (FDC) and they subsequently present antigenic peptides to T follicular helper (Tfh) cells to in order receive critical survival signals and undergo selection. Moreover, FDCs can have the ability to retain the native antigen to carry out the previous process, in addition to producing cytokines, such as BAFF, that help in the survival of the B cell.

In the LZ class-switch recombination (CSR) also occurs, where the constant region of the heavy chain of the antibody is changed, allowing B cells to produce IgG antibodies, IgA or IgE. This process diversififies the effector functions of antibodies, e.g., IgG can activate NK cells and phagocytes to eliminate cells infected by pathogens. In the final stage of the germinal center process, the centrocytes exit the GC as memory B cells or high-affifinity antibody-secreting plasma cells, but the molecular mechanisms that underlie this process are not yet entirely clear.

In the case of a plasma cell, the key is the activation of the Blimp1 master regulator (Prdm1), which, among many other functions, helps to stop the expression of transcription factors, such as Pax5 and Bcl6, responsible for the maintenance of B cells and the germinal center. For memory B cells, there is no established master regulator; however, a greater expression of Bach2 is fundamental. Plasma cells reside in the bone marrow and constitutively secrete antibodies; they do not possess BCR receptors, are not reactivated with antigenic re-exposition, and are responsible for producing serum antibodies that can last years after infection or vaccination. On the other hand, memory B cells express BCR but do not secrete antibodies constitutively. When they meet again with the antigen, they can reactivate and form GCs to produce antibodies with a greater affifinity; moreover, these cells can give rise to plasma cells and reside in circulation or peripheral lymphoid tissue.

The development of B cells can be seen in Figure 1. The antigen is processed in a dendritic cell to ensure its presentation and to produce cytokines. The dendritic cell within the lymph node presents antigen by class II MHC to naive T-lymphocytes, which differentiate in an effector or memory T-cell. The B cell can recognize native, unprocessed antigens with its BCR, but to perform a more specific response, the antigen needs to enter a germ center to be in contact with a follicular T cell that has already been presented with it, as this allows for it to be presented to the B cell and for the germ center reaction to begin, thereby producing specific antibodies, in this case, those against SARS-CoV-2 ².