piRNA: The Biomolecule Behind Immune-Privileged Body Organs

Every moment, a human body is exposed to several thousands of micro-organisms -- some of which are highly pathogenic, while others not so much. The immune system works relentlessly to prevent the entry of these microbes and the deadly infections they may cause. In case an infection does occur, the immune cells react immediately and secrete molecular weapons that can either kill or restrict the spread of the infection. However, this does not happen in all parts of the body.

Some body parts are hardwired not to react to an infection, an anomalous behavior termed as immune-privileged. Brain, eyes, placenta and testes are the most common immune-privileged organs.

The body considers these organs too important to be manipulated. For example, an infection of a testis may lead to change in the genomic constitution of the germ cells present inside it. Such conditions are often associated with infertility.

Some of the most common body mechanisms that ensure that these vital organs remain privileged are restricting the entry of infectious agents through the blood-brain and the blood-testis barrier, variable expression of MHC molecules and immune-suppressive cytokines. The most recent addition to the list is expression of piRNA molecules.

It has already been established that the piRNA molecules function by silencing the viral genomic sequences, thereby shielding the genome of germ line cells from undergoing manipulations. In other words, piRNA provides the ability to resist and tolerate viral infections without eliciting inflammatory responses, Phys.org reported.

According to a recent publication in eLife journal, researchers from the University of Rochester found that every time an infection occurs, the viral genome gets integrated with the host genome. The integrated portion gradually loses its pathogenicity due to accumulation of mutations. 

When the researchers tracked down avian leukosis virus infections in roosters, they found that the roosters use the old viral components present inside already infected testis to produce piRNA molecules. These piRNA molecules help in resisting further infections by related viral species, Science Daily reported.

Scientists believe that the research findings may be used for the advent of novel therapeutic strategies for the treatment and prevention of viral infections in chickens as well as human beings.