Mononuclear phagocytes and the intestinal tolerance

I already wrote a couple of entries about microbiota and the fact that our intestinal commensal bacteria do not stimulate aggressive response from the immune system. Most microorganisms share ligands recognized by innate receptors regardless of whether they are pathogens or symbionts. Therefore the way in which our immune system is viewed to operate – by recognition of conserved molecular patterns by antigen presenting cell populations and ensuing activation of immune response – does not explain well the “microbiota problem”. In fact the question how our body makes a distinction between “attack” vs. “hold on” options at mucosal surfaces still needs unraveling. I have found the report that makes an observation on the subject how intestinal tolerance could be maintained without compromising the need for appropriate response when endangered by infectious organisms. This publication suggests that gut-resident antigen presenting cells may be responsive to the presence of certain pathogen-indicating systems but not to ubiquitously present molecular conserved patterns.

The link:

Authors analyze cytokine release pattern (TNF-α, IL-6 and IL-1β) specific to a population coined as intestinal mononuclear phagocytes (iMPs), which are CD11b+ cells isolated from colonic/cecal lamina propria. Most iMPs bear macrophage marker F4/80. These intestinal antigen presenting cells do not respond by making cytokines to several TLR agonists or commensal bacteria but instead they are able to react to the pathogenic bacterium species – Salmonella. Their cytokine profile is also distinct from bone marrow-derived macrophages as they make only IL-1β but not TNF-α or IL-6. In contrast, bone marrow-derived macrophages produce uniformly TNF-α and IL-6 regardless of provided stimulation (TLR ligands or pathogenic bacterium).

IL-1β response by iMPs does not occur when cells are deficient for NLRC4 (cytosolic Nod-like receptor that forms part of inflammasome complex). It is also absent if Salmonella lacks type 3 secretion system (the apparatus that transports bacterial virulence factors into host cell) or flagellin. Developing this observation investigators provide molecular data that link the cleavage of pro-IL-1β into its active form with inflammasome activity (caspase-1 cleavage). Following the above finding, NLRC4-IL-1β axis is shown to be important for the protection against intestinal pathogenic bacterium in an in vivo model. Experimental infections with Salmonella that approximate human disease (by pretreating mice with streptomycin prior to infection) demonstrate worst survival rates for Nlrc4/ and Il1r/ mutants (however, this effect is strain-dependent as it takes place in BALB/c line but not C57BL/6 strain).

The major conclusion of this publication suggests the existence of a detection network that circumvents TLR signaling and relies on inflammasome activation by features unique to pathogens (like type 3 secretion system). However, I have a question that was not answered in the discussion part. Earlier this year the same group has shown that intestinal macrophages very similar to iMPs  (CD11b+F4/80+CD11c-/low) form the source of IL-1β secretion in response to microbiota (I wrote the entry about that publication few months ago – In the paper I am discussing today it is demonstrated that iMPs do not respond by making IL-1β  to commensal bacteria but can be stimulated only by T3SS-possessing pathogenic bacterium. Could it be caused by anatomical differences as this paper studies colonic/cecal lamina propria population and the former investigates processes in small intestine?

Follow-up note: I have contacted the principal investigator with questions concerning both papers. I received comments confirming that these results were caused by different anatomical locations (small vs. large intestine).

Franchi L, Kamada N, Nakamura Y, Burberry A, Kuffa P, Suzuki S, Shaw MH, Kim YG, & Núñez G (2012). NLRC4-driven production of IL-1β discriminates between pathogenic and commensal bacteria and promotes host intestinal defense. Nature immunology PMID: 22484733

Memory compartment regeneration in SIV infection does not rely on naive CD4 T cells

Among mechanisms hold responsible for the severe CD4 T cell depletion in AIDS are those that contribute to increased apoptosis rates for CD4 T cells (either infected or non-infected) and decreased CD4 T cells regeneration capacity. Pathogenic HIV/SIV clones are in their majority CCR5-tropic. CCR5 is expressed on tissue-resident effector memory CD4 T cell populations and not surprisingly these subsets are decimated during the primary HIV/SIV infection. However, the CCR5-negative central memory subset which resides in secondary lymphoid organs is relatively spared and may form the reservoir for the subsequent effector memory subsets regeneration. The publication I have found investigates details of memory CD4 T cells renewal in non-human primate model infected with SIV and makes an intriguing point about what is not needed to bring back depleted memory compartments.

The link:

The chief aim of this paper is to assess whether naïve CD4 T cells presence is needed for the memory compartment replenishment during pathogenic SIV infection. Authors employed rhesus macaques and subjected them to a number of manipulations before proper infection. Briefly, experimental groups have been either thymectomized or left intact and subsequently CD4 T cells in each group were depleted with anti-CD4 antibody following which animals were left to regenerate CD4 T cell counts. Such approach allows comparing regeneration of memory subsets and general disease outcome between two different conditions – complete lack vs. normal level of naïve CD4 T cells. If you have read my previous post, note that this report assumes that the thymus is the only place of significant naïve CD4 T cell development.

After complete rebuilding of CD4 T cells counts took place animals from each group were infected with SIVmac239. Then they underwent the long data collection period which included initial untreated infection, anti-retroviral therapy phase and treatment discontinuation in order for animals to develop AIDS-like symptoms. Investigators amassed data relating to multiple parameters – most notably they followed plasma viral load, memory CD4 T cells depletion levels (in blood and at mucosal lung surfaces) and the presence of anti-SIV adaptive immunity over the entire span of experiment. They also observed the disease progression and appearance of opportunistic infections in both groups. The summary conclusion of this study is that in the course of pathogenic SIV infection naïve CD4 cells presence is not necessary for the regeneration of memory CD4 T cells subsets. Additionally, the depletion of naive CD4 T cells do not seem to influence the disease advancement.

Major findings of this publication are very convincing and well supported with data. However, I would like to pose one question – it looks like the depletion of naive CD4 T cells in thymectomized group impairs the scope of adaptive anti-SIV response. For example, anti-SIV CD4 T cell responses (defined as the percentage of memory compartment specific to gag+pol+env+nef) never really start working. Thymectomized animals show much slower seroconversion rate and blunted anti-SIV CD8 T cell reactions as well. Yet the general disease outcome between both groups is remarkably similar at all studied stages. How to explain it?

Okoye, A., Rohankhedkar, M., Abana, C., Pattenn, A., Reyes, M., Pexton, C., Lum, R., Sylwester, A., Planer, S., Legasse, A., Park, B., Piatak, M., Lifson, J., Axthelm, M., & Picker, L. (2012). Naive T cells are dispensable for memory CD4+ T cell homeostasis in progressive simian immunodeficiency virus infection Journal of Experimental Medicine, 209 (4), 641-651 DOI: 10.1084/jem.20112071