22.4

Human MAbs

Taking a clue from production of mouse MAbs, initially to produce human MAbs,

various investigators made attempts to fuse human B cell with mouse myeloma cells.

However, mouse-human interspecies hybridomas preferentially segregate human

chromosomes. However, loss of human chromosome from mouse-human hybrids

is not random as human chromosomes 14 (encoding heavy chain) and 22 (encoding

λ-light chain) are preferentially retained. However, chromosome 2 encoding κ-light

chain is preferentially lost. In parallel, attempts have been made to develop suitable

human plasmacytoma and lymphoblastoid cell lines as fusion partner with human B

cell. Lymphocytes isolated from peripheral blood, bone marrow, spleen, tonsil, or

lymph node have been used for fusion. The isolated lymphocytes are stimulated

in vitro with pokeweed mitogen (PWM) or antigen or a combination of both.

Alternatively, Epstein-Bar virus (EBV) has also been used to transform human B

cell. EBV transformed human B cells divide, which can be grown in-vitro to produce

human antibodies. The main limitation of the EBV-transformed human B cell clones

is low amount of antibody produced and relative instability of these clones. The

relative instability and low amount of antibody produced by EBV-transformed cell

line can be overcome by their fusion with human plasmacytomas/lymphoblastoid

cell line. However, MAbs produced by using EBV-transformed human B cells have

safety concerns due to the possibility of minor contaminant of EBV nucleic acid in

the antibody formulation. Primarily, three different approaches are being used

currently to produce therapeutic human MAbs, which are briefly described below:

22.4.1 Production of Human MAbs Using Transgenic

Humanized Mice

Basically, this approach uses transgenic mice expressing heavy and light chains of

human instead of mouse antibody. In this direction, to begin with, transgenic mouse

lines have been developed in which the endogenous mouse heavy- and κ-light-chain

genes are inactivated and human transgenes encoding the heavy chain and κ-light

chain are introduced (Lonberg et al. 1994; Green et al. 1994). However, in these

transgenic mice, endogenous λ-light-chain locus of mouse has not been inactivated.

However, both IgG (Lonberg et al. 1994) and IgM (Green et al. 1994) human MAbs

recognizing specific target antigens have been produced using these transgenic mice.

In these transgenic mice, VDJ joining and somatic mutations as a function of

antibody class switch and their affinity maturation have also been documented

(Taylor et al. 1994). Subsequently, double “trans-chromosomic” mice have been

developed harboring human chromosome 2 and 14 fragments encompassing Ig

heavy chain locus and kappa light chain locus, whose endogenous IgH and Igkappa

loci were inactivated (Tomizuka et al. 2000). These mice were capable of producing

every subtype of fully human immunoglobulin, and on active immunization, anti-

body affinity maturation was also observed. Most of the transgenic mouse-derived

human MAbs have high binding affinity, which is comparable to mouse MAbs

22

Therapeutic Human Monoclonal Antibodies

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