1. Memory T cells may only reduce COVID-19 severity

The immune cells may mount a faster and stronger response upon exposure to the SARS-CoV-2 virus, but not totally prevent infection

At least six papers published in reputable journals — Cell, Nature, Science, and Science Immunology — have found that 20-50% of people who have not been infected with novel coronavirus (SARS-CoV-2) harbour memory T cells derived from previous exposures to common cold coronaviruses. The memory T cells were found to cross-react with SARS-CoV-2.

With Mumbai, Delhi, London and New York witnessing a peak weeks after the coronavirus blazed through the cities, people have wrongly assumed that the pre-existing memory T cells have been providing the much-needed protection from SARS-CoV-2 virus infection, leading to a mistaken assumption that herd immunity can be achieved when just 20-30% of the population is infected.

Reduce disease severity

“Pre-existing T cell immunity to SARS-CoV-2 could be relevant because it could influence COVID-19 disease severity. It is plausible that people with a high level of pre-existing memory CD4+ T cells that recognise novel coronavirus could mount a faster and stronger immune response upon exposure to the virus and thereby limit disease severity,” Alessandro Sette and Shane Crotty from La Jolla Institute for Immunology, La Jolla, California say in a Comment published in Nature Reviews Immunology.

Unlikely to prevent

“The memory T cells are extremely unlikely to prevent SARS-CoV-2 infections. That is just not what T cells generally do. We (Prof. Sette and I) speculate that they may reduce COVID-19 disease severity and prevent deaths,” Prof. Crotty says in an email to The Hindu. But in a paper published in Cell, a team led by Dr. Marcus Buggert from Karolinska Institutet, Stockholm, Sweden found robust memory T cell responses even in healthy people. Based on this, they noted that memory T cells indicate a “previously unanticipated degree of population-level immunity against COVID-19”, which contradicts the observation by others. In an email to The Hindu, he clarifies: “I completely agree with Prof. Crotty and Prof. Sette that T cells are not going to protect against infection [by SARS-CoV-2], but may prevent recurrent severe COVID-19. That’s what T cell immunity is all about.”

In a communication to The Hindu, Gagandeep Kang, Division of Gastrointestinal Sciences at CMC Vellore, echoed the views expressed by Prof. Sette and Prof. Crotty. “Memory T cells generated by seasonal coronavirus alone are very unlikely to prevent [SARS-CoV-2] infection. It is plausible that they play a role in reducing [COVID-19 disease] severity,” Prof. Kang says.

In a series of tweets, Prof. Crotty further clarified the misconstrued role of memory T cells in achieving herd immunity. “Even if our most optimistic speculations about cross-reactive T cell memory were found to be correct, it would mean that just as many people would get infected with SARS-CoV-2, but fewer would become severely ill and die from COVID-19,” Prof. Crotty says.

Killer T cells

Explaining how the memory T cells may help reduce the severity of the disease, virologist Dr. Shahid Jameel, CEO of DBT-Wellcome Trust India Alliance, says: “The cross-reactive memory T cells on activation would help in the development of plasma cells and thus antibody production, and in the development of killer T cells that would kill virus infected cells. The latter reduces the reservoirs of infection. This would most likely reduce disease severity.”

Herd immunity

A few researchers have also been mistaking pre-existing cross-reactive memory T cells to be playing a part in achieving herd immunity even when only a small percentage of the population is infected.

In a bid to clear the misconception regarding cross-reactive memory T cells and herd immunity, Prof. Crotty commented that “there are various tweets misinterpreting COVID-19 ‘pre-existing immunity’ and making dangerous claims about herd immunity”. He says cross-reactive immune memory, which avoids the word immunity, may sometimes get misinterpreted as “protective immunity”.

Dismissing the possibility of cross-reactive memory T cells producing herd immunity when only a small percentage of the population is infected, Dr Jameel says: “The [cross-reactive memory T cells] would not provide protective immunity from infection but may mitigate or reduce severity of disease. In fact, most vaccines also don’t prevent infection. They reduce or eliminate disease. How successful this is would depend upon the cross-reactivity at the T cell level.”

“Herd immunity is complicated, and most models have used calculations only for antibodies (and only stable antibodies at that). Hard to predict how much of a difference T cells would make,” Prof. Kang adds.

“T cell immunity will not generate herd immunity, but I truly believe that there could be an “unanticipated” degree of immunity against more severe forms of COVID-19 in the population… and maybe T cells play a role here, but so far this is based on associations,” Dr Buggert says.

2. Colliding haloes

Using Hubble space telescope, scientists studiying the halo of our nearest neighbouring galaxy, Andromeda, find that, at 1.3 million light years spread, the halo is stretching almost halfway up to the Milky Way. The galaxy’s halo appears to be already bumping into that of the Milky Way.

Milky Way

Location of Earth in Milky Way

Earth orbits the Sun, which is situated in the Orion Arm, one of the Milky Way’s spiral arms. Earth and the Sun are about 25,000 light-years away from the galactic center.

Size of Milky Way

The Stellar disk of the Milky Way is about 100,000 light-years across and 1,000 light-years thick. Based on current measurements, at least 90 percent of the mass in the Milky Way’s gravitational field is made up of dark matter, so the luminous stars, gas, and dust of the galaxy are embedded at the center of a huge, roughly spherical dark matter halo more than a million light-years across.

Blackhole at centre of milky way

There is an object at the center of the Milky Way called Sag-A* (Sagittarius A-star) which emits much more X-rays and radio waves than expected for a star-sized body.

After mapping the motions of stars near Sag A* for more than a decade, astronomers concluded that Sag A* is an invisible object that is more than three million times the mass of the Sun. This is a super massive black hole.

Movement of Earth within the Milky Way Galaxy

Earth (and the solar system) is moving through the Milky Way’s disk in a stable, roughly circular orbit around the galactic center. Our orbital velocity around the center of the Milky Way is about 200 kilometers per second. Even so, the Milky Way is so huge that one complete orbit takes about 250 million years.

Can we see the whole Milky Way?

Much of the galaxy is blocked from our view on Earth  due to the barriers created by dusty gas clouds blocking much of the light. Using infrared, microwave, and radio astronomy techniques, it is possible to penetrate much of this dusty fog. However, that too allows us only detect only half of stars and gas.

Neighbours of Milky Way

Milky way galaxy is one of the 54 galaxies in the “Local Group” of galaxies. The Local group itself is a part of a larger group called Virgo Supercluster. Virgo super cluster itself is a part of Laniakea Supercluster.

Within the local group, three largest galaxies viz. Milky Way, Andromeda and Triangulum have their own system of satellite small galaxies and clouds. For example, the satellite galaxies of Milky way include Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf, Ursa Minor Dwarf etc. Some of those galaxies, such as the Sagittarius dwarf galaxy, are almost in physical contact with the Milky Way’s outskirts.

The Andromeda Galaxy (aka. M31) is the closest large galaxy to the Milky Way. It can be seen from earth with naked eyes. This galaxy is considered to be slightly larger than Milky way and largest galaxy of the local group.

Important Facts About Andromeda

In the local cluster, the Andromeda is largest galaxy but may not be most massive. It is thought that Milky way has indeed more dark matter which makes is more massive.

Andromeda is nearest spiral galaxy from milky way. Both Andromeda and Milky way are approaching each other and it is thought that 4.5 billion years from this time, the Andromeda Galaxy and the Milky Way would collide and would form a giant elliptical galaxy.

Similarities between Andromeda and Milky Way

Both milkyway and Andromeda are spiral galaxies, both appear to be of same age and both have similar objects including a massive blackhole at the center. However, Andromeda is known to have a crowded double nucleus and its spiral arms are getting distorted by gravitational interactions with two satellite galaxies viz. M32 and M110.

Radio Galaxies

Radio galaxies are those ordinary-looking elliptical galaxies in which the radio wave emission far exceeds that of the galaxy’s visible light emission.

Large / Small Magellanic Clouds

The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC)  are two dwarf galaxies that are satellite galaxies of Milky way and orbit around Milkyway. Both of them are irregular galaxies. Both of them have forming stars at much faster rates than milkyway and so are important for the astronomers studying the formation and aging of stars and galaxies.

Interstellar medium

The interstellar medium is the matter that exists within, between and among the galaxies. Almost all of the interstellar medium is comprised of gas and microscopic dust particles.

Its density is very less, for example, interstellar medium in our region of the Milky Way galaxy has a density of about one atom of gas per cubic centimetre. By contrast, Earth’s atmosphere at sea level contains about 1019 gas molecules per cubic centimeter.