The discovery of hybridoma technology by Milstein and Kohler in 1975 allowed researchers to isolate specifically defined monoclonal antibodies (MAB). Although their potential in clinical use was immediately felt, it took more than a decade before a therapeutic monoclonal antibody became available in clinics. One of the major limitations was regarding the origin of MAB from mouse and administering to human beings. It acted as a foreign antibody, stimulating an immune reaction, and leading to adversities in therapeutics. Advancements in molecular biology later made it possible to develop human MAB in a way that is acceptable to the human immune systems.

Table 1. Therapeutic antibodies for treatment of cancer

Product	Therapeutic application	Manufacturer/ Inventor
Campath	B-cell lympholytic luekemia	Mellinium pharma Inc.
Gemtuzumab	Acute myeloid leukemia	Cell tech.
Herceptin	Matastatic breast cancer	Genetech.
Panorex	Colorectal cancer	Centocor  GSK
Rituximab	Non Hodgkin cancer	Genetech
Bexxar	Non Hodgkin lymphoma	Coraxia GSK
Edrecolomab	Colorectal cancer	Glaxo-wellcome
Alemtuzumab	Bull chronic lympholytic leukemia	ILEX Pharmaceuticals
Mylotrag	Acute myelogeneuos leukemia	Wyeth-Ayerst Research
IMC-C225	Head and Neck cancer	Imclone systems
Smartin 195	Acute myeloid leukemia
Mitomomab	Melanoma and Small cell lung cancer	Imclone systems

Generally, MAB is being used as invaluable reagents in diagnostics. In fact, they have played a major role in deciphering the functions of various bio-molecules in cryptic biosynthetic pathways. These have also become the reagents of choice for identification and characterization of tumor specific antigens and have become a valuable tool in the classification of cancer. Previously, as they were derived from murine sources, they had limited applications in therapeutic reagents. The repeated application to human subjects challenged the human immune system, leading to severe immune reactions. In order to circumvent the immunogenicity of murine antibodies, the development of humanized MAB, initialized with the strategy described below: -

* Immortalizing the human bursa derived cells.
* Chimeric antibodies comprising human and mice, in which a constant part from human antibodies are coined to a variable part of light and heavy chains from mouse in which 70% of the antibody comprising of human was retained.
* Humanized antibodies prepare by getting complementary determining regions of murine antibodies on to a human antibody framework, which has 90% resemblance to humans.
* Manipulation of CPR region towards the target antigen by linking the variable light and heavy chain segments to a single chain variable fragment.
* Use a transgenic map like HUMAB resulting in hybridomas producing 100X human MAB.
* Use trans-chromosomic mice having a part of human chromosome that contained complete germ-line clusters of heavy and light chain.

Table 2. Therapeutic antibodies approved for clinical practice.

Product	Therapeuticapplication	Manufacturer
OrthocloneOKT3	Kidneytransplant rejection	Orthobiotech
Reopro	Preventionof blood clot	Centocor
Remicade	Rhuetoidarthritis	Centocor
Zenapax	Preventionof acute kidney transplant	Reopro
Abciximab	Inhibitionof platelet function	Reopro

Traditionally, MAB have been produced as native molecules in murine hybridoma lines. However, there has been considerable interest in development of recombinant DNA expression of MAB. This would allow the production of humanized antibodies as well as spectrum of antibody derivatives and fusion proteins in a host species of choice. More recently, the production of antibodies in the milk (or other body fluids of transgenic animals and chicken eggs) has emerged as a promising alternative for fermentation based production systems. The main advantages of transgenic animals are potential high yields from renewable sources. However, the potential for scale up is limited and there are concerns that animals could harbor human pathogens such as viruses or prions, or that product could be contaminated with undesirable DNA sequence.

Table 3. Status of antibodies in clinical trails.

Product	Application	Manufacturer/inventor	Status
Avicidin	Marker for colorectal cancer	NoeRxMonsanato	Withdrawn from phase II clinical trails
CaroRx	Acts against Streptococcus advari mutants which causes tooth decay	Majk et.al.in 1999. Larrik, 2001.	Phase II clinical trails successful.
T84.66	Markers for epithelial cancers	Porrinet.al, 2000	Tested for tumor imaging and anti-tumor therapy
AntiHSV	Prevention of vaginal herpes simplex virus	Zeitilinet.al, 1998	Preventionin HSV-2 transmission in mouse
38C13	Potential for non-Hodgkin's lymphoma.	McCormicket. al , 1999	Prevents experimental induced lymphomas in mice
PIPP	Diagnosis and therapy of tumor producing HCG hormone in leidig's cells, pregnancy detection kit and contraception	Kathmicet.al, 2002	Developmental stage.

Plants have emerged recently as a convenient, safe and economical alternative main-stream expression systems for recombinant antibody production, which are based on large scale culture of microbes or animal cells. Several plant-derived antibodies have reached advanced stages of development and out of these, two have progressed to phase II clinical trials.

-- The authors Nagappa is with Pharmacy Group; Mukherjee & Konda Anusha are with BioSciences Group at Birla Institute of Technology and Science, Pilani.