Thread by @tony_breu

1/15
Why does clostridium difficile infection (CDI) cause marked leukocytosis?

Many of you have likely seen a new WBC >20k and wondered "could this patient have CDI?"

Are you right to wonder? If so, why?

2/
To start, is there a connection?

One of the earliest studies examined patients with WBC >30k. They reported the following rates of CDI:

🔹20% of all cases (excluding those with heme malignancy)
🔹34% of patients with an infectious etiology

pubmed.ncbi.nlm.nih.gov/12032893/

3/
In another study included 60 patients with unexplained leukocytosis (WBC >15k) and found:

⚡️58% had CDI⚡️

And: leukocytosis preceded recorded symptoms of colitis in half of the patients.

pubmed.ncbi.nlm.nih.gov/14599633/

4/
To understand how CDI leads to marked leukocytosis we must review where the "pool" of mature neutrophils resides.

At baseline, well over 90% of mature neutrophils are in the bone marrow.

pubmed.ncbi.nlm.nih.gov/12387736/

5/
Of the neutrophils in the blood, about 50% are circulating and 50% are marginated. This was demonstrated in 1961 in a study using radiolabeled neutrophils.

This marginated pool represents another source of neutrophils in leukocytosis.

ncbi.nlm.nih.gov/pubmed/13684959

6/
So, the source of neutrophils in CDI-mediated leukocytosis could be the bone marrow pool or the marginated pool.

Which is it?

7/
In a previous tweetorial, I noted that steroid-mediated leukocytosis is multifactorial with a large component coming from "demargination".

CDI is surely multifactorial too, but is demargination the main mechanism?

https://twitter.com/tony_breu/status/1053671045016813568?s=20

8/
At least one thing argues against demargination:

🔑Neutrophils appear in the colon within 1 hour of exposure to C. difficile toxin!

If demargination were the main cause of leukocytosis, you might expect a paucity of neutrophils in the target tissue.

ncbi.nlm.nih.gov/pubmed/7615182

9/
Is there evidence suggesting the bone marrow as the source of CDI-mediated leukocytosis?

Yes!

One clue: there is a surge in G-CSF levels in patients with CDI.

pubmed.ncbi.nlm.nih.gov/31211839/

10/
Why does an increase in G-CSF suggest the bone marrow as the source of the neutrophils in CDI-mediated leukocytosis?

🔑One of the main roles of G-CSF is to enhance the migration of neutrophils from the bone marrow to the blood!

pubmed.ncbi.nlm.nih.gov/12387736/

11/
Returning to the study in tweet 4, the investigators found that the blood pool of neutrophils increased after treatment with G-CSF.

Before: 0.8%
After: 9.8%

pubmed.ncbi.nlm.nih.gov/12387736/

12/
Before closing, another question: do other clostridial species have a similar propensity for marked leukocytosis?

If yes, is an increase in G-CSF also a mechanism?

13/
Yes and yes.

C. novyi and C. sordellii are known to cause marked leukocytosis with the latter occasionally leading to WBC in excess of 100k.

And, studies suggest a surge in G-CSF, just as seen in C. difficile.

pubmed.ncbi.nlm.nih.gov/26805011/

14/
What about C. perfringes?

It is unique in that neutrophils are NOT found in infected tissue, an observation first made in 1917.

This makes me wonder if the mechanism of leukocytosis in these patients is demargination and not an increase in G-CSF.

pubmed.ncbi.nlm.nih.gov/20768609/

15/15
➣ Clostridium difficile infection (CDI) is a common cause of unexplained leukocytosis
➣ CDI induces an increase in G-CSF
➣ G-CSF induces movement of neutrophils from the bone marrow pool to the circulating pool
➣ Some other clostridial species do the same