Day 2

Understanding the complex differentiation program of Hematopoietic Stem Cells towards a macrophage lineage

I’ve been reading a lot about how hematopoietic stem cells in the (mouse) bone marrow result in delineation of diverse myeloid fates such as a monocyte, macrophage, eosinophil, neutrophil or a dendritic cell. Considering the dynamic shifts in the BM due to peripheral triggers in mucosal and lymphoid tissues caused by pathology from aging, metabolic changes, infections, disease, damage and allergy - it is critical to thoroughly investigate the two-way traffic of the BM.

As HSCs in the mouse BM commit to the monocyte–macrophage lineage, they progressively trade a stem-like SLAM signature for a palette of myeloid and macrophage-defining markers. Classic immunology texts still depict this as a stepwise path from LT-HSC → ST-HSC/MPP → CMP → GMP → monocyte-lineage progenitors → monocytes → macrophages, which has now been refined by single-cell and fate-mapping studies. In flow cytometry, LT-HSCs are gated as Lin⁻ Sca-1⁺ c-Kit⁺ (LSK) CD150⁺ CD48⁻ CD34⁻, while myeloid-biased MPP2/3 remain LSK but become CD48⁺ with altered CD150/Flt3 expression. As cells drop Sca-1 and enter the Lin⁻ Sca-1⁻ c-Kit⁺ (LK) compartment, common myeloid progenitors (CMPs) appear as CD34⁺ CD16/32^lo, and granulocyte–monocyte progenitors (GMPs) as CD34⁺ CD16/32^hi, marking the first strong commitment to granulocyte/monocyte output. From here, monocyte-biased branches give rise to monocyte–dendritic progenitors (MDPs; Lin⁻ Sca-1⁻ c-Kit⁺ CD115⁺ Flt3⁺ CX3CR1⁺) and then common monocyte progenitors (cMoPs; Lin⁻ c-Kit⁺ CD115⁺ CX3CR1⁺ Ly6C⁺ Flt3⁻), a hierarchy consolidated in recent myeloid heterogeneity reviews. At this point, intracellular transcription factors such as IRF8, PU.1 (SPI1), and KLF4 are up-regulated, steering cells away from neutrophil and DC fates toward a monocyte–macrophage identity.

Within the bone marrow niche, cMoPs mature into Ly6C^hi CXCR4^hi pre-monocytes (preMos) and then Ly6C^hi CXCR4^lo monocytes, which are poised for egress to blood. These transitional preMos remain Lin⁻ CD115⁺ Ly6C^hi CXCR4^hi CCR2⁺ and still proliferate, whereas more mature Ly6C^hi monocytes acquire CD11b⁺ CX3CR1^lo and become strongly CCR2-dependent for exit—features nicely summarized in recent ontogeny reviews. As monocytes settle into tissue macrophage niches (or remain as marrow macrophages), they upregulate canonical macrophage markers such as F4/80 (Adgre1), CD64 (FcγRI), MerTK, and high CD11b, while maintaining CD115; at the transcriptional level, MafB, c-Maf, and sustained PU.1 consolidate macrophage identity.

Together, these surface and intracellular markers give you a practical flow-cytometry toolkit to track the moment when a generic myeloid progenitor “decides” to become a monocyte/macrophage in the mouse bone marrow.

I generated the image (below) using ChatGPT to make this more clear.. please tell me if things are either incorrect or outdated and I will refine this better.

*Image generated using ChatGPT with a lot of errors that I am working on fixing*

#DecemberAdventure

References:

https://www.labome.com/method/Macrophage-Markers.html?utm_source=chatgpt.com

https://www.sciencedirect.com/science/chapter/edited-volume/abs/pii/B9780120749034500327?utm_source=chatgpt.com

https://www.studocu.com/en-us/document/university-of-mississippi/immunology-and-serology/kuby-immunology-8th-ed-ch-2-lecture-slides-immune-system-cells-organs/138995937?utm_source=chatgpt.com

https://rupress.org/jem/article/213/11/2293/42007/CXCR4-identifies-transitional-bone-marrow

https://pmc.ncbi.nlm.nih.gov/articles/PMC10917229/?utm_source=chatgpt.com

https://www.nature.com/articles/s41586-024-07186-6?utm_source=chatgpt.com

https://www.mdpi.com/1422-0067/24/10/8757?utm_source=chatgpt.com

https://pmc.ncbi.nlm.nih.gov/articles/PMC9256773/?utm_source=chatgpt.com

https://pmc.ncbi.nlm.nih.gov/articles/PMC5726802/?utm_source=chatgpt.com

https://www.nature.com/articles/s41586-024-07186-6?utm_source=chatgpt.com

https://cuvas.edu.pk/cuvas_libraries/ebooks/Owen%20Kuby%20Immunology%207th%20%20Ed.%20%282013%29.pdf?utm_source=chatgpt.com

December Adventure

Take a journey with me as I start a #DecemberAdventure starting tomorrow for 25 days. This has been on my list of things to do for many years, especially as I read these posts on Mastodon/BSKY each year by programmers who pick a project and chip at it each day for the advent calendar.

How this is going to work is simple: I will try to write a blog post each day for 25 days of December starting tomorrow that talks about something I am doing/learning from a published paper, textbook chapter, a seminar.. heck even my group meeting at work. It can also be something I am trying to do for a manuscript/project. It’ll be pretty broad just so I can muster up the motivation to follow through.

So, come on this adventure with me!

(Day 0, #DecemberAdventure)

Day 1

Rewriting Immunity: Epigenetic Memory and the Rise of Trained Type-2 Responses

I have been fascinated with type 2 immunity for a good part of a decade and decided to re-read this fabulous review by Franziska Hartung and Julia Esser-von Bieren, Trained immunity in type 2 immune responses. Hartung and Bieren beautifully synthesize emerging evidence that trained immunity plays a substantial role in shaping type 2 immune responses such as helminth infection, allergy, and asthma. Definition of trained immunity according to leading experts in the field, is “long-term functional reprogramming of innate immune cells, which is evoked by exogenous or endogenous insults and which leads to an altered response towards a second challenge after the return to a non-activated state.” This type of epigenetic and metabolic programming can act on long lived innate cells such as NK cells, macrophages and monocytes, HSCs. More recently, there's been suggestions of innate memory potential on eosinophils and ILCs as well.

In thinking about this review and rather than regurgitating the highlights..I’d like to just think out loud- some of the broad strokes about type 2 immunity.

One thing that has always struck out to me is how anything that doesn’t fit in type 1 immunity is bucketed as a type 2 immunity. But, the reality is that type 2 immunity has a wide spectrum from allergy, autoimmunity, asthma, helminths infections, metabolic diseases as well as fibrosis. Having said that, you can imagine how complex the triggers and the resulting consequences can be.

For instance, a helminth infection and an asthmatic response may result in eosinophilia, yet the consequences of this exuberant eosinophilia can be incredibly host protective during helminths infection in mounting a robust type 2 immune response against a secondary helminth infection.. while maintenance of such eosinophils can also be immensely pathogenic and inflammatory in asthma. This complexity is telling of how underappreciated the diverse type 2 immune responses really are.

This is almost a conversation starter and we can let this incubate and circle back to it and let it evolve over this month. But, as far as the review goes.. one of the many enigmatic and open questions that still remain in the field of trained immunity and type 2 immunity - I hadn’t thought much about this one:

Do IL-33-responsive progenitors in the bone marrow form a long-term memory niche that sustains chronic asthma even in the absence of allergen exposure. If so, can this explain why there is a persistent inflammation in CRSwNP and NSAID-exacerbated diseases. Why don’t you chew on that for a bit and I’ll think of something else to talk about tomorrow.