Science for Everyone #2: The Tale of the Great Pneumococcal Invasion

For this installment of the Science series, I wanted to share a story I wrote as part of a final in a microbiology class a while back. I really had a good time writing it and thought you guys might find it entertaining. I give you The Tale of the Great Pneumococcal Invasion which pitted the macrophages and other cells of the immune system against the pesky invaders, Streptococcus pneumoniae!

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A long time ago in an organ far away, I was part of great battle. Invaders came and seemed to grow out of nowhere. Our defenses were put to the test and we nearly lost our home! It was a hard fought campaign but in the end my kin and I emerged victorious and fended off the bacterial invaders! Come you little monocytes, gather around Grandpa Mac and hear the tale of the Great Pneumococcal Invasion.

It started like any other day in the lungs. The air was moist and humid with a warm breeze blowing. Some time had passed since the last assault on our home and we had grown comfortable. I was getting ready to call it a day and was travelling down my bronchiole into my alveolus when I noticed a strange film on the walls. What I first thought was an isolated spot seemed to keep going deeper into the depths of the lung. As I got continued deeper into the cavernous passage, I began to smell the metallic tinge of blood and as I turned a corner I saw them. They were small, strange oval shaped creatures wearing a polysaccharide armor. They didn’t seem too harmful, they were tiny after all. How bad could they be?

My complacency got the better of me and it wasn’t until I heard the screams around me that I knew something was wrong. I turned quickly to see what was causing such a commotion to find it was the invaders making the fuss. Some of them seemed to be vomiting and rolling in what was expelled (autolysins) from their tiny bodies and melted their armor. Upon touching the juice they discharged, the invaders shrieked as holes were formed in their tiny bodies spewing an even more corrosive liquid (Pneumolysin) that began to rip open the walls around them. I knew then this was no commensal bacteria we were dealing with. I rushed the tiny trespassers and attempted to surround them with my pseudopods and engulf them like previous intruders, but their armor prevented me from doing so. What was this strange, sugary substance that was stopping me from engulfing the tiny assailants?! Protected by this substance, I watched as more lysed themselves releasing they’re pneumolysin and tearing a hole in the walls of my home.

As blood began to seep in I knew this was serious. If I couldn’t eat them, I couldn’t degrade them with my enzymes and release my cytokines to call for reinforcements. It was then that Danny the Dendritic cell came stumbling along and tried to help. Despite his best efforts he couldn’t phagocytize them either. More blood came in and we began to worry when we saw complement proteins. They always get the job done surely they could help. But as they began to split and attack, C3b bounced off their armor too. We began to panic when Danny managed to attach to one of the intruders with one of his Toll-Like Receptors. I knew our only chance was to call in reinforcements so I told Danny to run to the nearest lymph node leaving a trail of cytokines and chemokines along the way to lead an army of neutrophils and other lymphoid cells back to our location. The CD4+ T cells and B cells that lived in the lymph nodes may have been immature but we needed all the help we could get!

After he departed, I looked for anything to help in the fight and was able to find pieces of one of the raiders and immediately ate them. The power I gained allowed me to call to other nearby macrophages. Due to the increasing chaos around me I had failed to notice there were far more of the creatures now. They grew much faster than anything I’d ever seen! More of my macrophage brothers arrived and we found some of these aliens were edible! As we rolled around eating and picking up pieces of the invaders we sent out more cytokines to call any nearby neutrophils that may be able to help. In the process we were gaining strength and becoming activated by the same cytokines. We were making progress but these foreign attackers were multiplying faster than we could take them out and they began to overwhelm our local forces. They were gaining ground and I was beginning to lose hope.

As more of the tiny thieves showed themselves, I watched as my brothers began to be buried under the constantly growing pile of foreign bodies. There seemed to be no way to stop these creatures as they overwhelmed our defenses. When it appeared that nothing we were doing was going to stop them was when I heard it. It was the most beautiful sound these old ears ever heard. Danny was yelling to hang on that had made his way back from the lymph nodes! And he had brought reinforcements! But these weren’t the immature T cells I was expecting. No, these were big strong CD4+ Th cells! As they rushed into battle they tossed me a macrophage’s favorite cytokine, Interferon γ. My strength grew as I felt myself becoming “angry.” These trespassers aren’t going to like me when I’m angry. Gathering my strength I watched as more reinforcements came from lymph nodes. The Th cells brought along their cousins the B cells who had grown into antibody producing plasma cells! Once Danny was able to wake up the T cells, they stimulated their B cell brethren who produced antibodies to the antigens that Danny showed them! We may win this battle yet!

The B cells began throwing their antibodies at the invaders allowing my macrophage teammates to recognize and eat the attacking bacteria! As the other macrophages joined me in my rage, we began increasing our supplies of our secret weapons: H2O2, O2, and NO. We bombarded the invading pathogens with our reactive oxygen species and began to beat back the invaders. The formerly compromised complement began to see the foreign creatures as they were covered with the antibodies released from the B cells and the bacteria began bursting and gushing all around us as the complement deposited membrane attack complexes into their membranes causing the attackers to lose all their membrane integrity. It was at this point that the neutrophils arrived and began releasing their toxic granules to clean up the remaining attacking forces and, tired from the battle, I shook hands with a nearby Treg cell and was calmed to my previously unstimulated state.

A long time has passed since this great fight and we haven’t had an established infection from these foreigners since. Along with the antibody producing plasma cells, the immature B cells also grew into memory B cells. These memory cells remember the armor and other proteins the invading pathogens used breach our defenses making sure that we won’t fall for their tricks again. If those tricksters decide to come around again the memory cells will sound the alarm and we will be able to attack and fight them off before they establish themselves and threaten our existence again. We fought long and hard that day but we emerged victorious. Thanks to the teamwork and determination showed by all the different parts of the immune system we were able to beat back these tiny titans and bring back the lungs to perfect working shape!

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All images are from Biowars: The Battle Rages Within comic books.