SFC/EM Investigación

Half-life of T cells: bases of LDI posology

I couldn’t resist to study the subject about half-lives of Lymphocytes, to see whether or not the 7 weeks rule is correct. It took me reading like 20 articles, and some books on immunology, as it is a very complicated question, that even as of today seems not having been properly answered. Well, I made a quick research, and a quick reading, so my conclusions might not be acurate. A proper review of the literature would require a few days at least. Still I think I got a fairly clear idea about it:

First we need to differentiate between "half-life", that, when applied to any field, it means the same: the time for what you are measuring to go down to half. On the other hand, the "life-span" is the time it takes for what you are measuring to die or disappear. Also we have the term “disappearance rate”, that is the time between the first measurement—when the amount of what you are measuring is usually maximum -- and the last measurement—in which you cannot detect what you are measuring.

While I initially tried to get conclusions from various results from different studies, I realized that it was very difficult to do so, as when they say CD4+ T cells, each study measured different cells, subsets, etc. I have therefore decided to study in depth one very solid study in humans that really measured what we need, and after explaining this study, we can try to compare the conclusions I drew with other paper's results.

I reviewed the following study:
http://jem.rupress.org/content/200/2/255.abstract

(I studied the whole text, but the abstract is quite complete). The following text is a very illustrative explanation I copied from the article:

(…)Total peripheral blood CD4+CD3+ cells in the subjects studied: were subdivided by cell sorting into four populations: 21% TCM (CD45R0+CCR7+), 12% TEM (CD45R0+CCR7-), 64% Naïve (CD45R0- (or CD45RA+)CCR7+), and 3% CD45R0-CCR7-). (…)

(…) A major subdivision among memory T cells is that between central–memory (TCM) and effector memory (TEM) cells. These subpopulations, which are identified among human CD45R0+ T cells as being CCR7+CD62L+ or CCR7-CD62L-, respectively, exhibit distinct functional properties. TCM cells enter LNs through high endothelial venules and recirculate primarily between blood and lymph, whereas TEM cells migrate preferentially from the blood into peripheral tissues such as the lung or intestinal mucosa (9–11). In addition, TEM cells express effector activity(e.g., cytolytic activity or secretion of cytokines such as IFN-gamma, IL-4, or IL-5) more rapidly than TCM cells upon restimulation with antigen. (…)

(…)On this basis, TCM and TEM cells have been proposed to play complementary roles in the secondary response to infection, with TEM cells providing a rapid effector response at sites of pathogen entry and TCM cells serving as an expanded pool of precursors that can proliferate in the secondary lymphoid organs to rapidly generate large numbers of effector cells. (…)

(…) Overall, the average proliferation rates for TCM and TEM cells were 1.5 and 4.7% per day, respectively. From p, the average intermitotic time (doubling) for cells within that population (t2) can be calculated, yielding values of 48 d for TCM cells and 15 d for TEM cells (…)

(…) Disappearance rates, though more variable, were also higher for TEM than TCM cells in five of the six subjects. On average, the disappearance rates were 4.1% per day for TCM cells and 11.3% per day for TEM cells (equivalent to half-lives [t1/2] for disappearance of the cells of 17 and 6 d, respectively).(…) (it is evident that the contribution of cell division to maintaining the naive CD4+ T cell pool in young adults is minimal.

(…)In contrast the CD45R0- (orCD45RA+)CCR7+ naive CD4+ T cell population was found to be much longer lived, being labeled at a rate of only 0.2% per day (corresponding to an intermitotic time of approximately 1 yr). These data indicate that human CD4+ TEM cells constitute a short-lived cell population that requires continuous replenishment in vivo.

(…) Overall, the relative quiescence of naive CD4+T cells and rapid turnover of memory–phenotype cells reported here are in agreement with the general pattern of kinetic behavior that has been observed for these cells in many different species. Our data show that TCM and TEM CD4+ T cells in humans have distinct rates of turnover, with TEM cells typically being replaced at a much faster rate than TCM cells. These results imply that an almost continuous input of new cells is required to maintain the TEM CD4+T cell population, which could have important implications for vaccination given that TEM may serve to provide a rapid effector response to reinfection. it could be argued that the shorter lifespan observed for TEM cells in the blood is simply a consequence of these cells having an ability to migrate rapidly and irreversibly into tissues. However, if one assumes that the number of TEM cells in tissues is constant (rather than increasing) under steady-state conditions, turnover of cells in the blood ultimately reflects what is occurring in the total TEM population. Therefore, it is evident from the rapid rate of production that there is also a rapid rate of disappearance among TEM cells.(...)

In SHORT: this is my explanation from the above data:

The naïve T cells' half-life (the ones we have to “train” with LDI) is very long, being the time it takes for the cells to double their numbers of almost a year. So it seems that actually CD4+ T naïve cells do not multiply much and they live about a year; they go from the thymus to the Lymph nodes, where they are converted into memory/effector cells (Th1, Th2, Th17, Th23, T regs…), and these effectors cells are the ones which undertake a very rapid turnover. Thus, we see how the 2 classes of memory/effector cells (TEM and TCM) double their numbers within 15 and 48 days respectively, while their “disappearance half-lives” is just 6 and 17 days respectively, meaning that their span-live is 12 and 34 days respectively (assuming a linear decline, that might not be accurate, but I think we can work with it after seeing the almost linear decline of the graphic shown in the paper).

So, when we hear that we need to separate LDI/LDA dosis 7 weeks from one another (when taking the same antigen either when we’ve reached a core dose, or when we have flared from it and we have to back down the next dose), and that these 49 days have to be with the “half-life” of Lymphocytes, which lymphocytes are we really talking about? Alright, it seems that the span-life of T naïve cells is very long, so their levels in blood are steady, and so there will be always enough naive T cells to enter into the lymph nodes and wait there to be converted.

Then, which cells make us need to wait in order to not suffer an “overdose” that causes a flare?: Those cells that are responsible for the flare, right?, well, these are the Tcm and the Tem (these are the T cells that are already specific for 1 single epitope of an antigen, and go around the body “patrolling”. So, when we take a dose of LDI 2 things might occur:

1. Antigen-Presenting Cells (APC) in the mucosae uptake the antigens we give with LDI/A, then travel to the lymph nodes and present the antigens to naïve T cells, which will be converted into Th1, Th2, Th17, Tregs, etc., according to the kind and the amount of antigens we give. Then they go to the bloodstream. If we give a proper antigen at a proper dose, they will mainly form Tregs (inducing this way tolerance) and also proper T effector cells towards the antigen—for instance Th1 and Th17 cells for Lyme. This is what we want to achieve with LDI/LDA.

2. IF specific and improper T effector/memory cells—TEM and TCM (called this way because they have the memory of recognizing a given specific antigen and then rapidly mount an effector response when this occurs) encounter these antigen (Ags) (e.g. APCs presents to them the Ags before reaching the naïve T cells in the Lymph nodes), they will readily become activated and will mount the response they have been born to develop: in Lyme disease this is going to be an improper inflammatory response, Th2 dominant, etc. So, we will flare if this happens.

Alright, how to make the process number 1 to prevail? We need the newly formed specific TEM and TCM cells improperly set to die and therefore we’ll give APCs more chances to induce naïve T cells to be converted into the Tregs, Th1 and Th17 cells we need.

The pool of improper TEM and TCM cells in chronic infections seem to be steady. They are constantly dying and being renewed in the lymphatic nodes because the inflammatory signals and the endogenous antigens of borrelia (or any other chronic infection) make this process chronic. What we need is to slowly make the T naïve cells to be converted into the proper cells instead of into the inflammatory T cells which causes our symptoms, and it seems we have this chance when we give low doses of antigens, even in the chronic presence of T inflammatory cells. But, when we have given too stronger a dose, we have created temporarily a lot of new T inflammatory cells, which sicken us and also create a kind of competence for naïve T cells to be converted into the proper T cells we need. The same seem to happen when we are giving a core dose: we have “saturated” the capacity of lymph nodes to create new cells, but in this case it is because we have created proper T cells (Tregs, etc.), and they are going all around the body making us better by switching the pathological immune profile for a healthy one. It seems we need this newly created proper TEM and TCM cells to also die in order for us to be able to create new ones. The reason is unclear to me as it is very difficult to find data on this, but I guess it has to be with the amount of antigens given (think that there will always be inflammatory T cells also circulating throughout the body, so the only way to bypass them is to play with the dosages).

In SHORT, according to this study, the most solid I have found out of many, we’ll need to wait an average of 48 days between doses of LDI (as its rules state), what fits with the 49 days Dr. Ty recommends.

¿Does other studies support this? Just a quick review, taking from the studies quoted below. The first study estimates an average lifespan of TEM and TCM CD4+ cells of 164 days. Unfortunately they might be including the span-life of T naïve cells, that is very long, so this number might be higher than the real one. In monkeys, another study finds 64 days to be the mean half-life of total CD4+ cells (that equals to 128 days of life-span); and here they are fore sure counting the naïve T cells, so the number might be about a 64% higher than the probable real one (because T naïve cells account for the approximately 64% of cells of total CD4+ cells circulating in blood). Very interesting is the fact that if we take this 64% out of T naïve cells, we get: 0.36x 128=46 days. Similarly, another study shows that the life-span of Th1 or Th2 cells (effector cells) is 50 days. In the same line, the next paper founds the half-life of total CD4+ cells in blood to be 84 days, which equals a span-life of 168 days. If we take away the 64% of naïve T cells, we get 60 days. Finally, the last study, although they specify that they are measuring memory and effector CD4+ cells, and find their half-life to be of 155 days, given the similar results to the previous ones, I think they are also counting some very little fraction of naïve T cells (as explained in the first study I reviewed), which would make this number to be, again, higher than the true one—due to the very low rate of division and the long span-lives of T naïve cells.

IN SHORT: it seems 7 weeks to be accurate according to the literature I have revived. Also, even if the span-lives of the T memory/effect cells were a bit higher for some people, it wouldn’t matter, as the decline of these cells is exponential. Last but not the least, we see that the 7 week rule seem to work for most fellows, so together with the fact that it seems to also fit with several different studies, my layman conclusion is that we can be quite confident with this 7 weeks rule.

Hope it helps,
Sergio

STUDIES QUOTED:


http://www.bloodjournal.org/content/122 ... ecked=true:

(…)mouse CD4+ and CD8+ effector/memory T cells have average turnover rates of 0.068 (95% confidence interval [CI], 0.065-0.088) and 0.050 (95% CI, 0.045-0.085) per day (Figure 3E), corresponding to average life spans of 15 days (95% CI, 11-15 days) for CD4+, and 20 days (95% CI, 12-22 days) for CD8+ effector/memory T cells (…)In humans, the current best estimates are that memory CD4+ and CD8+ T cells live 164 and 157 days, respectively(…)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC111455/:

(…)High turnover rate of CD4+ T lymphocytes in sooty mangabeys. In order to ascertain whether the dynamics of normal lymphocyte turnover differed between sooty mangabeys and rhesus macaques, we investigated the relative turnover rates of different lymphocyte subsets in sooty mangabeys. Similar to findings in rhesus macaques (6), the turnover rate of B lymphocytes in sooty mangabeys was significantly higher than that of CD4+ and CD8+ T lymphocytes (Fig. 3A). The estimated mean half-life of total B lymphocytes was 41 days compared to 63 days for CD4+ T lymphocytes and 93 days for CD8+ T lymphocytes (Table 2). (…)

http://www.sciencedirect.com/science/ar ... 4096000365:

(…)Dynamics of the systemic scaled concentrations (x(r) and Y(T)) of the antigen-specific Thl/Th2 cells. Since a T unit corresponds to a T-cell life-span (approximately 50 days),(…)
https://www.ncbi.nlm.nih.gov/pubmed/9883844:
(…)In HIV-1-seronegative subjects, the fractional replacement rates of circulating CD4+ and CD8+ T cells were 0.008 ± 0.005 per day and 0.009 ± 0.013 per day, respectively (Fig. 3a and Table), corresponding to half-lives of 87 and 77 days, respectively (…) The shorter half-life of circulating CD4+ T cells in the HAART group may reflect complex underlying population dynamics. It is possible that activated ‘memory’ CD4+ T cells are dying at an accelerated rate while ‘naive’ cells are being produced in greater numbers, for example (…)

http://www.pnas.org/content/105/16/6115.full:

(…)The turnover rates of memory CD4 and CD8 T cells were found to be ≈10-fold higher, i.e., p = 0.0045 and 0.0028 per day, corresponding to half-lives of 155 and 244 days for memory CD4 and CD8 T cells (…)